Document how to tell git to not launch a pager

Signed-off-by: Steven Grimm <koreth@midwinter.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -417,7 +417,9 @@ other
 	See gitlink:git-merge[1]
 
 'GIT_PAGER'::
-	This environment variable overrides `$PAGER`.
+	This environment variable overrides `$PAGER`. If it is set
+	to an empty string or to the value "cat", git will not launch
+	a pager.
 
 'GIT_FLUSH'::
 	If this environment variable is set to "1", then commands such