Documentation: update-index: -z applies also to --index-info

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-update-index.txt

@@ -144,8 +144,8 @@ you will need to handle the situation manually.
         Report what is being added and removed from index.
 
 -z::
-	Only meaningful with `--stdin`; paths are separated with
+	Only meaningful with `--stdin` or `--index-info`; paths are
-	NUL character instead of LF.
+	separated with NUL character instead of LF.
 
 \--::
 	Do not interpret any more arguments as options.