docs: clarify git diff modes of operation

docs: clarify git diff modes of operation

It is an oversimplification to say that we can take
"[<commit> [<commit>]]", as it really depends on what
options have been given. Instead, let's list the major modes
of operation separately, as we do in other manpages.

This patch also adjusts the text immediately after the
synopsis to match the lines given in the synopsis.

For git-difftool, which has the same issue, let's refer the
user to the git-diff manpage rather than spelling it all out
again.

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