[bug#39501] [PATCH] doc: Clarify how sudo and guix interact when updating

[Jakub Kądziołka]

* doc/guix.texi (After System Installation): Explicitly explain the
  mechanics of how sudo differs from a login shell.

Some recent discussions on IRC suggest that the existing wording wasn't
entirely clear.
---
 doc/guix.texi | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/doc/guix.texi b/doc/guix.texi
index 710cf97673..9f758706f6 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -72,6 +72,7 @@ Copyright @copyright{} 2019 Guillaume Le Vaillant@*
 Copyright @copyright{} 2020 Leo Prikler@*
 Copyright @copyright{} 2019, 2020 Simon Tournier@*
 Copyright @copyright{} 2020 Wiktor Żelazny@*
+Copyright @copyright{} 2020 Jakub Kądziołka@*
 
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -2439,6 +2440,11 @@ your system includes the latest security updates 
(@pxref{Security Updates}).
 Note that @command{sudo guix} runs your user's @command{guix} command and
 @emph{not} root's, because @command{sudo} leaves @code{PATH} unchanged.  To
 explicitly run root's @command{guix}, type @command{sudo -i guix @dots{}}.
+
+The difference matters here, because @command{guix pull} updates
+the @command{guix} command and package definitions only for the user it is ran
+as. This means that if you choose to use @command{guix system reconfigure} in
+root's login shell, you'll need to @command{guix pull} separately.
 @end quotation
 
 Join us on @code{#guix} on the Freenode IRC network or on
-- 
2.25.0