net: add missing kdoc for struct genl_multicast_group::flags

Multicast group flags were added in commit 4d54cc32 ("mptcp: avoid
lock_fast usage in accept path"), but it missed adding the kdoc.

Mention which flags go into that field, and do the same for
op structs.

Link: https://lore.kernel.org/r/20220809232012.403730-1-kuba@kernel.org


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