mailmap: Update comments for with format and more detalis

Without having first read the git-shortlog man-page, the format
of .mailmap may not be immediately obvious. Add comments with pointers
to the man-page, along with other details.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/202008080013.58EBD83@keescook


Signed-off-by: Jonathan Corbet <corbet@lwn.net>