doc/checkout: refer to git-branch(1) as appropriate

doc/checkout: refer to git-branch(1) as appropriate

Most of description for the branch creation options is
simply cut and paste from git-branch. There are two reasons
to fix this:

  1. It can grow stale with respect to what's in "git
     branch" (which it is now is).

  2. It is not just an implementation detail, but rather the
     desired mental model for the command that we are using
     "git branch" here. Being explicit about that can help
     the user understand what is going on.

It also makes sense to strip the branch creation options
from the synopsis, as they are making it a long,
hard-to-read line. They are still easily discovered by
reading the options list, and --track is explicitly
referenced when branch creation is described.

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