kernel-doc: fix new warning in net/sock.h

Fix new kernel-doc warning:

Warning(include/net/sock.h:372): No description found for parameter 'sk_cgrp_prioidx'
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -226,6 +226,7 @@ struct cg_proto;
   *	@sk_ack_backlog: current listen backlog
   *	@sk_max_ack_backlog: listen backlog set in listen()
   *	@sk_priority: %SO_PRIORITY setting
+  *	@sk_cgrp_prioidx: socket group's priority map index
   *	@sk_type: socket type (%SOCK_STREAM, etc)
   *	@sk_protocol: which protocol this socket belongs in this network family
   *	@sk_peer_pid: &struct pid for this socket's peer