Document AF_UNIX control messages in unix(4) only, not split between
unix(4) and recv(2).
Also, warn about LOCAL_CREDS effective uid/gid fields, since the
write could be from a setuid or setgid program (with the explicit SCM_CREDS
and LOCAL_PEERCRED, the credentials are read at such a time that it can
be assumed that the process intends for them to be used in this context).