[PATCH v3] 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 <liu.denton@gmail.com>
---
Thanks for catching the typo, Phillip. I can't believe I missed that in
such a simple patch.

Range-diff against v2:
1:  ed9f16c30a ! 1:  381b2ca134 git-diff.txt: document return code of `--no-index`
    @@ Documentation/git-diff.txt: two blob objects, or changes between two files on di
      	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 `--no-exit`.
    ++	controlled by Git. This form implies `--exit-code`.
      
      'git diff' [<options>] --cached [<commit>] [--] [<path>...]::
      

 Documentation/git-diff.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)