Documentation: driver-api: update kernel connector

This patch changes:
1) Fix typo in kernel connector documentation.
	s/cn_netlink_send_multi/cn_netlink_send_mult/
2) update definition of struct cn_msg

Signed-off-by: Wang Long <w@laoqinren.net>
Link: https://lore.kernel.org/r/1595229852-114071-1-git-send-email-w@laoqinren.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>