net/sock.h: add missing kernel-doc notation

Add missing kernel-doc notation to struct sock:

Warning(include/net/sock.h:324): No description found for parameter 'sk_peer_pid'
Warning(include/net/sock.h:324): No description found for parameter 'sk_peer_cred'
Warning(include/net/sock.h:324): No description found for parameter 'sk_classid'
Warning(include/net/sock.h:324): Excess struct/union/enum/typedef member 'sk_peercred' description in 'sock'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>