doc: show usage of branch description

doc: show usage of branch description

The branch description will be included in 'git format-patch
--cover-letter' and in 'git pull-request' emails. It can also
be used in the automatic merge message. Tell the reader.

While here, clarify that the description may be a multi-line
explanation of the purpose of the branch's patch series.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>