gnt-instance(8) one more batch-create update

Document the new nics list, as an alternative to the one nick which you
can create with the old mac, ip, mode, link/bridge keys.

Also specify that 'bridge' is still accepted as well.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>