doc: Improve UEFI/BIOS bootloader documentation.

Partly fixes <https://bugs.gnu.org/30312>.

* doc/guix.texi (Preparing for Installation): Add note on how to choose
between UEFI and BIOS.
(Using the Configuration System)[Bootloader]: New subsubsection.
(Bootloader Configuration): Expound on the bootloader type and target.
* gnu/system/examples/desktop.tmpl: Switch to UEFI.
* gnu/system/examples/bare-bones.tmpl: Explicitly mention "legacy" and
"BIOS" in the comments.

doc/guix.texi

@@ -8636,6 +8636,17 @@ must have the @code{esp} flag set.  E.g., for @command{parted}:
 parted /dev/sda set 1 esp on
 @end example
 
+@quotation Note
+@vindex grub-bootloader
+@vindex grub-efi-bootloader
+Unsure whether to use EFI- or BIOS-based GRUB?  If the directory
+@file{/sys/firmware/efi} exists in the installation image, the you should
+probably perform an EFI installation, using @code{grub-efi-bootloader}.
+Otherwise you should use the BIOS-based GRUB, known as
+@code{grub-bootloader}.  @xref{Bootloader Configuration}, for more info on
+bootloaders.
+@end quotation
+
 Once you are done partitioning the target hard disk drive, you have to
 create a file system on the relevant partition(s)@footnote{Currently
 GuixSD only supports ext4 and btrfs file systems.  In particular, code
@@ -8964,6 +8975,27 @@ Below we discuss the effect of some of the most important fields
 fields), and how to @dfn{instantiate} the operating system using
 @command{guix system}.
 
+@unnumberedsubsubsec Bootloader
+
+@cindex legacy boot, on Intel machines
+@cindex BIOS boot, on Intel machines
+@cindex UEFI boot
+@cindex EFI boot
+The @code{bootloader} field describes the method that will be used to boot
+your system.  Machines based on Intel processors can boot in ``legacy'' BIOS
+mode, as in the example above.  However, more recent machines rely instead on
+the @dfn{Unified Extensible Firmware Interface} (UEFI) to boot.  In that case,
+the @code{bootloader} field should contain something along these lines:
+
+@example
+(bootloader-configuration
+  (bootloader grub-efi-bootloader)
+  (target "/boot/efi"))
+@end example
+
+@xref{Bootloader Configuration}, for more information on the available
+configuration options.
+
 @unnumberedsubsubsec Globally-Visible Packages
 
 @vindex %base-packages
@@ -9074,8 +9106,7 @@ management, power management, and more, would look like this:
 @include os-config-desktop.texi
 @end lisp
 
-@cindex UEFI
-A graphical UEFI system with a choice of lightweight window managers
+A graphical system with a choice of lightweight window managers
 instead of full-blown desktop environments would look like this:
 
 @lisp
@@ -20697,21 +20728,30 @@ The type of a bootloader configuration declaration.
 The bootloader to use, as a @code{bootloader} object. For now
 @code{grub-bootloader}, @code{grub-efi-bootloader},
 @code{extlinux-bootloader} and @code{u-boot-bootloader} are supported.
+
+@vindex grub-efi-bootloader
 @code{grub-efi-bootloader} allows to boot on modern systems using the
-@dfn{Unified Extensible Firmware Interface} (UEFI).
+@dfn{Unified Extensible Firmware Interface} (UEFI).  This is what you should
+use if the installation image contains a @file{/sys/firmware/efi} directory
+when you boot it on your system.
+
+@vindex grub-bootloader
+@code{grub-bootloader} allows you to boot in particular Intel-based machines
+in ``legacy'' BIOS mode.
 
 Available bootloaders are described in @code{(gnu bootloader @dots{})}
 modules.
 
 @item @code{target}
 This is a string denoting the target onto which to install the
-bootloader.  The exact interpretation depends on the bootloader in
-question; for @code{grub-bootloader}, for example, it should be a device
-name understood by the bootloader @command{installer} command, such as
-@code{/dev/sda} or @code{(hd0)} (for GRUB, @pxref{Invoking
-grub-install,,, grub, GNU GRUB Manual}).  For
-@code{grub-efi-bootloader}, it should be the path to a mounted EFI file
-system.
+bootloader.
+
+The interpretation depends on the bootloader in question.  For
+@code{grub-bootloader}, for example, it should be a device name understood by
+the bootloader @command{installer} command, such as @code{/dev/sda} or
+@code{(hd0)} (@pxref{Invoking grub-install,,, grub, GNU GRUB Manual}).  For
+@code{grub-efi-bootloader}, it should be the mount point of the EFI file
+system, usually @file{/boot/efi}.
 
 @item @code{menu-entries} (default: @code{()})
 A possibly empty list of @code{menu-entry} objects (see below), denoting

gnu/system/examples/bare-bones.tmpl

@@ -10,8 +10,9 @@
   (timezone "Europe/Berlin")
   (locale "en_US.utf8")
 
-  ;; Assuming /dev/sdX is the target hard disk, and "my-root" is
-  ;; the label of the target root file system.
+  ;; Boot in "legacy" BIOS mode, assuming /dev/sdX is the
+  ;; target hard disk, and "my-root" is the label of the target
+  ;; root file system.
   (bootloader (bootloader-configuration
                 (bootloader grub-bootloader)
                 (target "/dev/sdX")))

gnu/system/examples/desktop.tmpl

@@ -11,11 +11,11 @@
   (timezone "Europe/Paris")
   (locale "en_US.utf8")
 
-  ;; Assuming /dev/sdX is the target hard disk, and "my-root"
-  ;; is the label of the target root file system.
+  ;; Use the UEFI variant of GRUB with the EFI System
+  ;; Partition mounted on /boot/efi.
   (bootloader (bootloader-configuration
-                (bootloader grub-bootloader)
-                (target "/dev/sdX")))
+                (bootloader grub-efi-bootloader)
+                (target "/boot/efi")))
 
   ;; Specify a mapped device for the encrypted root partition.
   ;; The UUID is that returned by 'cryptsetup luksUUID'.