doc: Add a 'Upgrading Guix' section to the 'Installation' chapter.

This follows a discussion regarding issue #36785 (see:
https://bugs.gnu.org/36785).

* doc/guix.texi (Upgrading Guix): New section.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -513,6 +513,7 @@ ready to use it.
 * Setting Up the Daemon::       Preparing the build daemon's environment.
 * Invoking guix-daemon::        Running the build daemon.
 * Application Setup::           Application-specific setup.
+* Upgrading Guix::              Upgrading Guix and its build daemon.
 @end menu
 
 @node Binary Installation
@@ -1761,6 +1762,40 @@ invoke the actual linker with this new set of arguments.  You can instruct the
 wrapper to refuse to link against libraries not in the store by setting the
 @code{GUIX_LD_WRAPPER_ALLOW_IMPURITIES} environment variable to @code{no}.
 
+@node Upgrading Guix
+@section Upgrading Guix
+
+@cindex Upgrading Guix, on a foreign distro
+
+To upgrade Guix, run:
+
+@example
+guix pull
+@end example
+
+@xref{Invoking guix pull}, for more information.
+
+@cindex upgrading Guix for the root user, on a foreign distro
+@cindex upgrading the Guix daemon, on a foreign distro
+@cindex @command{guix pull} for the root user, on a foreign distro
+
+On a foreign distro, you can upgrade the build daemon by running:
+
+@example
+sudo -i guix pull
+@end example
+
+@noindent
+followed by (assuming your distro uses the systemd service management
+tool):
+
+@example
+systemctl restart guix-daemon.service
+@end example
+
+On Guix System, upgrading the daemon is achieved by reconfiguring the
+system (@pxref{Invoking guix system, @code{guix system reconfigure}}).
+
 @c TODO What else?
 
 @c *********************************************************************