doc: Suggest using 'guix pull' before 'guix system reconfigure'.

* doc/guix.texi (Invoking guix system): Suggest running 'guix pull'
  before the first use of 'guix system reconfigure'.

doc/guix.texi

@@ -4343,6 +4343,13 @@ It also adds a GRUB menu entry for the new OS configuration, and moves
 entries for older configurations to a submenu---unless
 @option{--no-grub} is passed.
 
+@c The paragraph below refers to the problem discussed at
+@c <http://lists.gnu.org/archive/html/guix-devel/2014-08/msg00057.html>.
+It is highly recommended to run @command{guix pull} once before you run
+@command{guix system reconfigure} for the first time (@pxref{Invoking
+guix pull}).  Failing to do that you would see an older version of Guix
+once @command{reconfigure} has completed.
+
 @item build
 Build the operating system's derivation, which includes all the
 configuration files and programs needed to boot and run the system.