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 <[e-mail]>
Reviewed-by: Bagas Sanjaya <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/userspace-api/netlink/intro.rst

···
   | ** optionally extended ACK                 |
   ----------------------------------------------
 
+Note that some implementations may issue custom ``NLMSG_DONE`` messages
+in reply to ``do`` action requests. In that case the payload is
+implementation-specific and may also be absent.
+
 .. _res_fam:
 
 Resolving the Family ID