Clean up documentation of AF_UNIX control messages.

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).

Reviewed by: wblock
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D9298