git.txt: add missing info about --git-dir command-line option

Unlike other environment variables (e.g. GIT_WORK_TREE,	GIT_NAMESPACE),
the Documentation/git.txt file did not mention that the GIT_DIR
environment variable can also be set using the --git-dir command line
option.

Signed-off-by: Manlio Perillo <manlio.perillo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -650,6 +650,7 @@ git so take care if using Cogito etc.
 	If the 'GIT_DIR' environment variable is set then it
 	specifies a path to use instead of the default `.git`
 	for the base of the repository.
+	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