Most of the git-diff-* documentation used [<common diff options>]
instead of [--diff-options], so make that change in git-diff and
git-format-patch.

In addition, git-format-patch didn't include the meanings of the diff
options.
Signed-off-by: NBrian Gernhardt <benji@silverinsanity.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>