Documentation improvements for the description of short format. Incorporates the detailed explanation from Jeff King in <20100410040959.GA11977@coredump.intra.peff.net> and fixes the bug noted by Junio C Hamano in <7vmxxc1i8g.fsf@alter.siamese.dyndns.org>. Signed-off-by: Eric S. Raymond <esr@thyrsus.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>