recv.2: Mention SOCK_SEQPACKET in MSG_TRUNC flag description

recv.2: Mention SOCK_SEQPACKET in MSG_TRUNC flag description

unix_seqpacket_recvmsg() calls unix_dgram_recvmsg() which handles
MSG_TRUNC. This has been the case since the handling was added in
9f6f9af7694ede6314bed281eec74d588ba9474f; see net/unix/af_unix.c:

static int unix_seqpacket_recvmsg([...])
{
        [...]
	return unix_dgram_recvmsg(iocb, sock, msg, size, flags);
}

The sequential-packet socket type seems to have been left out from the
description by an oversight.

Signed-off-by: Vladislav Ivanishin <vlad@ispras.ru>
Signed-off-by: Alejandro Colomar <alx@kernel.org>