doc: Replace invalid uses of @var with @code.

* doc/guix.texi: Replace @var with @code in cases where we refer to a
Scheme variable and not to a metasyntactic variable.

doc/guix.texi

@@ -6445,7 +6445,7 @@ Connect to the daemon over the Unix-domain socket at @var{uri} (a string).  When
 extra space on the file system so that the garbage collector can still
 operate should the disk become full.  Return a server object.
 
-@var{file} defaults to @var{%default-socket-path}, which is the normal
+@var{file} defaults to @code{%default-socket-path}, which is the normal
 location given the options that were passed to @command{configure}.
 @end deffn
 
@@ -10357,7 +10357,7 @@ configuration options.
 The @code{packages} field lists packages that will be globally visible
 on the system, for all user accounts---i.e., in every user's @code{PATH}
 environment variable---in addition to the per-user profiles
-(@pxref{Invoking guix package}).  The @var{%base-packages} variable
+(@pxref{Invoking guix package}).  The @code{%base-packages} variable
 provides all the tools one would expect for basic user and administrator
 tasks---including the GNU Core Utilities, the GNU Networking Utilities,
 the GNU Zile lightweight text editor, @command{find}, @command{grep},
@@ -10418,7 +10418,7 @@ customize them.  To do this, use @code{modify-services} (@pxref{Service
 Reference, @code{modify-services}}) to modify the list.
 
 For example, suppose you want to modify @code{guix-daemon} and Mingetty
-(the console log-in) in the @var{%base-services} list (@pxref{Base
+(the console log-in) in the @code{%base-services} list (@pxref{Base
 Services, @code{%base-services}}).  To do that, you can write the
 following in your operating system declaration:
 
@@ -10442,7 +10442,7 @@ following in your operating system declaration:
 
 This changes the configuration---i.e., the service parameters---of the
 @code{guix-service-type} instance, and that of all the
-@code{mingetty-service-type} instances in the @var{%base-services} list.
+@code{mingetty-service-type} instances in the @code{%base-services} list.
 Observe how this is accomplished: first, we arrange for the original
 configuration to be bound to the identifier @code{config} in the
 @var{body}, and then we write the @var{body} so that it evaluates to the
@@ -10473,15 +10473,15 @@ This example refers to the @file{/boot/efi} file system by its UUID,
 as returned by the @command{blkid} command.
 
 @xref{Desktop Services}, for the exact list of services provided by
-@var{%desktop-services}.  @xref{X.509 Certificates}, for background
+@code{%desktop-services}.  @xref{X.509 Certificates}, for background
 information about the @code{nss-certs} package that is used here.
 
-Again, @var{%desktop-services} is just a list of service objects.  If
+Again, @code{%desktop-services} is just a list of service objects.  If
 you want to remove services from there, you can do so using the
 procedures for list filtering (@pxref{SRFI-1 Filtering and
 Partitioning,,, guile, GNU Guile Reference Manual}).  For instance, the
 following expression returns a list that contains all the services in
-@var{%desktop-services} minus the Avahi service:
+@code{%desktop-services} minus the Avahi service:
 
 @example
 (remove (lambda (service)
@@ -10600,7 +10600,7 @@ A procedure that returns an initial RAM disk for the Linux
 kernel.  This field is provided to support low-level customization and
 should rarely be needed for casual use.  @xref{Initial RAM Disk}.
 
-@item @code{firmware} (default: @var{%base-firmware})
+@item @code{firmware} (default: @code{%base-firmware})
 @cindex firmware
 List of firmware packages loadable by the operating system kernel.
 
@@ -10635,7 +10635,7 @@ device, provided that the necessary device mapping and file system are
 also specified.  @xref{Mapped Devices} and @ref{File Systems}.
 
 @item @code{users} (default: @code{%base-user-accounts})
-@itemx @code{groups} (default: @var{%base-groups})
+@itemx @code{groups} (default: @code{%base-groups})
 List of user accounts and groups.  @xref{User Accounts}.
 
 If the @code{users} list lacks a user account with UID@tie{}0, a
@@ -10655,11 +10655,11 @@ For instance, a valid value may look like this:
                           (activate-readline)")))
 @end example
 
-@item @code{issue} (default: @var{%default-issue})
+@item @code{issue} (default: @code{%default-issue})
 A string denoting the contents of the @file{/etc/issue} file, which is
 displayed when users log in on a text console.
 
-@item @code{packages} (default: @var{%base-packages})
+@item @code{packages} (default: @code{%base-packages})
 The set of packages installed in the global profile, which is accessible
 at @file{/run/current-system/profile}.
 
@@ -10678,7 +10678,7 @@ causes @command{guix system} to fail.
 The name of the default locale (@pxref{Locale Names,,, libc, The GNU C
 Library Reference Manual}).  @xref{Locales}, for more information.
 
-@item @code{locale-definitions} (default: @var{%default-locale-definitions})
+@item @code{locale-definitions} (default: @code{%default-locale-definitions})
 The list of locale definitions to be compiled and that may be used at
 run time.  @xref{Locales}.
 
@@ -10687,12 +10687,12 @@ The list of GNU@tie{}libc packages whose locale data and tools are used
 to build the locale definitions.  @xref{Locales}, for compatibility
 considerations that justify this option.
 
-@item @code{name-service-switch} (default: @var{%default-nss})
+@item @code{name-service-switch} (default: @code{%default-nss})
 Configuration of the libc name service switch (NSS)---a
 @code{<name-service-switch>} object.  @xref{Name Service Switch}, for
 details.
 
-@item @code{services} (default: @var{%base-services})
+@item @code{services} (default: @code{%base-services})
 A list of service objects denoting system services.  @xref{Services}.
 
 @cindex essential services
@@ -11557,7 +11557,7 @@ more.
 
 This is the default value of the @code{services} field of
 @code{operating-system} declarations.  Usually, when customizing a
-system, you will want to append services to @var{%base-services}, like
+system, you will want to append services to @code{%base-services}, like
 this:
 
 @example
@@ -12054,7 +12054,7 @@ Whether to authorize the substitute keys listed in
 (@pxref{Substitutes}).
 
 @vindex %default-authorized-guix-keys
-@item @code{authorized-keys} (default: @var{%default-authorized-guix-keys})
+@item @code{authorized-keys} (default: @code{%default-authorized-guix-keys})
 The list of authorized key files for archive imports, as a list of
 string-valued gexps (@pxref{Invoking guix archive}).  By default, it
 contains that of @code{@value{SUBSTITUTE-SERVER}} (@pxref{Substitutes}).
@@ -12062,7 +12062,7 @@ contains that of @code{@value{SUBSTITUTE-SERVER}} (@pxref{Substitutes}).
 @item @code{use-substitutes?} (default: @code{#t})
 Whether to use substitutes.
 
-@item @code{substitute-urls} (default: @var{%default-substitute-urls})
+@item @code{substitute-urls} (default: @code{%default-substitute-urls})
 The list of URLs where to look for substitutes by default.
 
 @item @code{max-silent-time} (default: @code{0})
@@ -12223,7 +12223,7 @@ allows users to use the mouse in the console, notably to select, copy,
 and paste text.
 
 The value for services of this type must be a @code{gpm-configuration}
-(see below).  This service is not part of @var{%base-services}.
+(see below).  This service is not part of @code{%base-services}.
 @end defvr
 
 @deftp {Data Type} gpm-configuration
@@ -14732,7 +14732,7 @@ services that users typically expect on a machine with a graphical
 environment and networking:
 
 @defvr {Scheme Variable} %desktop-services
-This is a list of services that builds upon @var{%base-services} and
+This is a list of services that builds upon @code{%base-services} and
 adds or adjusts services for a typical ``desktop'' setup.
 
 In particular, it adds a graphical login manager (@pxref{X Window,
@@ -14747,7 +14747,7 @@ name service switch service configured to be able to use @code{nss-mdns}
 (@pxref{Name Service Switch, mDNS}).
 @end defvr
 
-The @var{%desktop-services} variable can be used as the @code{services}
+The @code{%desktop-services} variable can be used as the @code{services}
 field of an @code{operating-system} declaration (@pxref{operating-system
 Reference, @code{services}}).
 
@@ -16756,7 +16756,7 @@ Data type representing the configuration of opensmtpd.
 @item @code{package} (default: @var{opensmtpd})
 Package object of the OpenSMTPD SMTP server.
 
-@item @code{config-file} (default: @var{%default-opensmtpd-file})
+@item @code{config-file} (default: @code{%default-opensmtpd-file})
 File-like object of the OpenSMTPD configuration file to use.  By default
 it listens on the loopback network interface, and allows for mail from
 users and daemons on the local machine, as well as permitting email to
@@ -24363,7 +24363,7 @@ to the @code{packages} field of the @code{operating-system} declaration
 @code{nss-certs}, which is a set of CA certificates provided as part of
 Mozilla's Network Security Services.
 
-Note that it is @emph{not} part of @var{%base-packages}, so you need to
+Note that it is @emph{not} part of @code{%base-packages}, so you need to
 explicitly add it.  The @file{/etc/ssl/certs} directory, which is where
 most applications and libraries look for certificates by default, points
 to the certificates installed globally.
@@ -24457,7 +24457,7 @@ want is to have @code{.local} host lookup working.
 Note that, in this case, in addition to setting the
 @code{name-service-switch} of the @code{operating-system} declaration,
 you also need to use @code{avahi-service-type} (@pxref{Networking Services,
-@code{avahi-service-type}}), or @var{%desktop-services}, which includes it
+@code{avahi-service-type}}), or @code{%desktop-services}, which includes it
 (@pxref{Desktop Services}).  Doing this makes @code{nss-mdns} accessible
 to the name service cache daemon (@pxref{Base Services,
 @code{nscd-service}}).