doc/checkout: split checkout and branch creation in synopsis These can really be thought of as two different modes, since the "<branch>" parameter is treated differently in the two (in one it is the branch to be checked out, but in the other it is really a start-point for branch creation). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>