Doc User-Manual: Patch cover letter, three dashes, and --notes

Show that git format-patch can have a cover letter, include patch
commentary below the three dashes, and notes can also be
included.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Doc format-patch: note ability to edit before sending

Signed-off-by: Philip Oakley <philipoakley@iee.org>