diff options documentation: refer to --diff-filter in --name-status

git diff --name-status outputs letters, but the meaning of those letters
is documented elsewhere. Add a note to make the manpage more intuitive.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -75,7 +75,8 @@ endif::git-format-patch[]
 	Show only names of changed files.
 
 --name-status::
-	Show only names and status of changed files.
+	Show only names and status of changed files. See the description
+	of the `--diff-filter` option on what the status letters mean.
 
 --color::
 	Show colored diff.