Documentation: describe the format of messages with inline patches

Documentation: describe the format of messages with inline patches

Add a DISCUSSION section to the "git format-patch" manual to encourage
people to send patches in a form that can be applied by "git am"
automatically.  There are two such forms:

 1. The default form in which most metadata goes in the mail header
    and the message body starts with the patch description;

 2. The snipsnip form in which a message starts with pertinent
    discussion and ends with a patch after a "scissors" mark.

The example requires QP encoding in the "Subject:" header intended for
the mailer to give the reader a chance to reflect on that, rather than
being startled by it later.  By contrast, in-body "From:" and
"Subject:" lines should be human-readable and not QP encoded.

Inspired-by: Jim Meyering <jim@meyering.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Improved-by: Junio C Hamano <gitster@pobox.com>
Improved-by: Drew Northup <drew.northup@maine.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>