net: add missing kdoc for struct genl_multicast_group::flags

net: add missing kdoc for struct genl_multicast_group::flags

[ Upstream commit 5c221f0af68cfa9edcffd26ba6dbbc4b7ddb1700 ]

Multicast group flags were added in commit 4d54cc32112d ("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>
Stable-dep-of: e03781879a0d ("drop_monitor: Require 'CAP_SYS_ADMIN' when joining "events" group")
Signed-off-by: Sasha Levin <sashal@kernel.org>