docs/format-patch: mention handling of merges

docs/format-patch: mention handling of merges

Format-patch doesn't have a way to format merges in a way that can be
applied by git-am (or any other tool), and so it just omits them.
However, this may be a surprising implication for users who are not well
versed in how the tool works. Let's add a note to the documentation
making this more clear.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>