doc: Mention value /var to localstatedir option.

* README (Installing Guix from Guix): Mention value /var to localstatedir option.
* doc/contributing.texi (Building from Git): Likewise.
* doc/guix.texi (Requirements): Likewise.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

README

@@ -63,7 +63,8 @@ To do so:
   - Re-run the 'configure' script passing it the option
     '--localstatedir=/somewhere', where '/somewhere' is the 'localstatedir'
     value of the currently installed Guix (failing to do that would lead the
-    new Guix to consider the store to be empty!).
+    new Guix to consider the store to be empty!).  We recommend to use the
+    value '/var'.
 
   - Run "make", "make check", and "make install".
 

doc/contributing.texi

@@ -94,7 +94,8 @@ more information.
 Then, run @command{./configure} as usual.  Make sure to pass
 @code{--localstatedir=@var{directory}} where @var{directory} is the
 @code{localstatedir} value used by your current installation (@pxref{The
-Store}, for information about this).
+Store}, for information about this).  We recommend to use the value
+@code{/var}.
 
 Finally, you have to invoke @code{make check} to run tests
 (@pxref{Running the Test Suite}).  If anything

doc/guix.texi

@@ -792,8 +792,9 @@ When configuring Guix on a system that already has a Guix installation,
 be sure to specify the same state directory as the existing installation
 using the @code{--localstatedir} option of the @command{configure}
 script (@pxref{Directory Variables, @code{localstatedir},, standards,
-GNU Coding Standards}).  The @command{configure} script protects against
-unintended misconfiguration of @var{localstatedir} so you do not
+GNU Coding Standards}).  Usually, this @var{localstatedir} option is
+set to the value @file{/var}.  The @command{configure} script protects
+against unintended misconfiguration of @var{localstatedir} so you do not
 inadvertently corrupt your store (@pxref{The Store}).
 
 @node Running the Test Suite