doc: './pre-inst-env guix pull' won't upgrade local source tree.

* doc/contributing.texi (Running Guix Before It Is Installed): Clarify
  './pre-inst-env guix pull' won't upgrade local source tree.
* doc/guix.texi (Invoking guix pull): Mention ~/.config/guix/latest.

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