git-diff.txt: document return code of --no-index

Within diff_no_index(), we have the following:

	revs->diffopt.flags.exit_with_status = 1;

	...

	/*
	 * The return code for --no-index imitates diff(1):
	 * 0 = no changes, 1 = changes, else error
	 */
	return diff_result_code(&revs->diffopt, 0);

Which means when `git diff` is run in `--no-index` mode, `--exit-code`
is implied. However, the documentation for this is missing in
git-diff.txt.

Add a note about how `--exit-code` is implied in the `--no-index`
documentation to cover this documentation blindspot.

Signed-off-by: Denton Liu <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

Documentation/git-diff.txt

@@ -36,7 +36,7 @@ two blob objects, or changes between two files on disk.
 	running the command in a working tree controlled by Git and
 	at least one of the paths points outside the working tree,
 	or when running the command outside a working tree
-	controlled by Git.
+	controlled by Git. This form implies `--exit-code`.
 
 'git diff' [<options>] --cached [<commit>] [--] [<path>...]::