format-patch documentation: mention the special case of showing a single commit

format-patch documentation: mention the special case of showing a single commit

Even long timers seem to have missed that "format-patch -1 $commit" is a
much simpler and more obvious way to say "format-patch $commit^..$commit"
from the current documentation (and an example "format-patch -3 $commit"
to get three patches).  Add an explicit instruction in a much earlier part
of the documentation to make it easier to find.

Signed-off-by: Junio C Hamano <gitster@pobox.com>