bisect: document command line arguments for "bisect start"

bisect: document command line arguments for "bisect start"

The syntax commonly used for alternatives is --opt-(a|b), not
--opt-{a,b}.

List bad/new and good/old consistently in this order, to be
consistent with the description for "git bisect terms".  Clarify
<term> to either <term-old> or <term-new> to make them consistent
with the description of "git bisect (good|bad)" subcommands.

Suggested-by: Matthieu Moy <git@matthieu-moy.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>