SubmittingPatches: mention doc-diff

SubmittingPatches: mention doc-diff

We already advise people to make sure their documentation
formats correctly. Let's point them at the doc-diff script,
which can help with that.

Let's also put a brief note in the script about its purpose,
since that otherwise can only be found in the original
commit message. Along with the existing -h/usage text,
that's hopefully enough for developers to make use of it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>