net: add missing kdoc for struct genl_multicast_group::flags

Multicast group flags were added in commit 4d54cc3 ("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>

include/net/genetlink.h

@@ -11,6 +11,7 @@
 /**
  * struct genl_multicast_group - generic netlink multicast group
  * @name: name of the multicast group, names are per-family
+ * @flags: GENL_* flags (%GENL_ADMIN_PERM or %GENL_UNS_ADMIN_PERM)
  */
 struct genl_multicast_group {
 	char			name[GENL_NAMSIZ];
@@ -116,7 +117,7 @@ enum genl_validate_flags {
  * struct genl_small_ops - generic netlink operations (small version)
  * @cmd: command identifier
  * @internal_flags: flags used by the family
- * @flags: flags
+ * @flags: GENL_* flags (%GENL_ADMIN_PERM or %GENL_UNS_ADMIN_PERM)
  * @validate: validation flags from enum genl_validate_flags
  * @doit: standard command callback
  * @dumpit: callback for dumpers
@@ -137,7 +138,7 @@ struct genl_small_ops {
  * struct genl_ops - generic netlink operations
  * @cmd: command identifier
  * @internal_flags: flags used by the family
- * @flags: flags
+ * @flags: GENL_* flags (%GENL_ADMIN_PERM or %GENL_UNS_ADMIN_PERM)
  * @maxattr: maximum number of attributes supported
  * @policy: netlink policy (takes precedence over family policy)
  * @validate: validation flags from enum genl_validate_flags