Fix usage for soreceive(9) - uio must always be non-NULL.

MFC after:	1 week

share/man/man9/socket.9

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd December 14, 2006
+.Dd April 12, 2013
 .Dt SOCKET 9
 .Os
 .Sh NAME
@@ -208,12 +208,19 @@ Data may be retrieved directly to kernel or user memory via the
 argument, or as an mbuf chain returned to the caller via
 .Fa mp0 ,
 avoiding a data copy.
-Only one of the
+The
 .Fa uio
-or
-.Fa mp0
-pointers may be
+must always be
 .Pf non- Dv NULL .
+If
+.Fa mp0
+is
+.Pf non- Dv NULL ,
+only the
+.Pf uio_resid
+of
+.Fa uio
+is used.
 The caller may optionally retrieve a socket address on a protocol with the
 .Dv PR_ADDR
 capability by providing storage via