genetlink: add missing kdoc for validation flags

Validation flags are missing kdoc, add it.

Fixes: ef6243acb4 ("genetlink: optionally validate strictly/dumps")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/genetlink.h

@@ -138,6 +138,7 @@ genl_dumpit_info(struct netlink_callback *cb)
  * @cmd: command identifier
  * @internal_flags: flags used by the family
  * @flags: flags
+ * @validate: validation flags from enum genl_validate_flags
  * @doit: standard command callback
  * @start: start callback for dumps
  * @dumpit: callback for dumpers