check-ignore: clarify treatment of tracked files

By default, check-ignore does not list tracked files at all since
they are not subject to ignore patterns.

Make this clearer in the man page.

Reported-by: Guilherme <guibufolo@gmail.com>
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-check-ignore.txt

@@ -21,6 +21,9 @@ the exclude mechanism) that decides if the pathname is excluded or
 included.  Later patterns within a file take precedence over earlier
 ones.
 
+By default, tracked files are not shown at all since they are not
+subject to exclude rules; but see `--no-index'.
+
 OPTIONS
 -------
 -q, --quiet::