net: improve and fix netlink kdoc

Subsequent patch will render the kdoc from
include/uapi/linux/netlink.h into Documentation.
We need to fix the warnings. While at it move
the comments on struct nlmsghdr to a proper
kdoc comment.

Link: https://lore.kernel.org/r/20220819200221.422801-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>