doc: Mention that guix.el works out-of-the-box on GSD.

* doc/emacs.texi (Emacs Initial Setup): Mention that it works
  out-of-the-box on GSD.

doc/emacs.texi

@@ -24,6 +24,12 @@ guix package}).  Specifically, ``guix.el'' makes it easy to:
 @node Emacs Initial Setup
 @subsection Initial Setup
 
+On the Guix System Distribution (@pxref{GNU Distribution}), ``guix.el''
+is ready to use, provided Guix is installed system-wide, which is the
+case by default.  So if that is what you're using, you can happily skip
+this section and read about the fun stuff.
+
+If you're not yet a happy user of GSD, a little bit of setup is needed.
 To be able to use ``guix.el'', you need to install the following
 packages: