format-patch: give an overview of what a "patch" message is
The text says something called a "patch" is prepared one for each
commit, it is suitable for e-mail submission, and "am" is the
command to use it, but does not say what the "patch" really is.
The description in the page also refers to the "three-dash" line,
but it is unclear what it is, unless the reader is given a more
detailed overview of what the "patch" is.
Add a brief paragraph to give an overview of what the output looks
like.
Signed-off-by: Junio C Hamano <gitster@pobox.com>