Documentation: clarify %(contents:XXXX) doc

Documentation: clarify %(contents:XXXX) doc

Let's avoid a big dense paragraph by using an unordered
list for the %(contents:XXXX) format specifiers.

While at it let's also make the following improvements:

  - Let's not describe %(contents) using "complete message"
    as it's not clear what an incomplete message is.

  - Let's improve how the "subject" and "body" are
    described.

  - Let's state that "signature" is only available for
    tag objects.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>