Doc: add more detail for git-format-patch

Doc: add more detail for git-format-patch

In git-format-patch.txt, we were missing some key user information.
First of all, document the special value of `--base=auto`.

Next, while we're at it, surround option arguments with <> and change
existing names such as "Message-Id" to "message id", which conforms with
how existing documentation is written.

Finally, document the `format.outputDirectory` config and change
`format.coverletter` to use camel case.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>