git.txt: remove stale comment regarding GIT_WORK_TREE

Official support for specifying --work-tree/GIT_WORK_TREE without
--git-dir/GIT_DIR was added with v1.7.4-rc3~2^2~2.  Update description
of GIT_WORK_TREE to reflect this.

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -654,9 +654,7 @@ git so take care if using Cogito etc.
 	The '--git-dir' command-line option also sets this value.
 
 'GIT_WORK_TREE'::
-	Set the path to the working tree.  The value will not be
-	used in combination with repositories found automatically in
-	a .git directory (i.e. $GIT_DIR is not set).
+	Set the path to the root of the working tree.
 	This can also be controlled by the '--work-tree' command line
 	option and the core.worktree configuration variable.