docs: netlink: add NLMSG_DONE message format for doit actions

In case NLMSG_DONE message is sent as a reply to doit action, multiple
kernel implementation do not send anything else than struct nlmsghdr.
Add this note to the Netlink intro documentation.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20231201180154.864007-1-jiri@resnulli.us


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