net: update documentation for WriteMsgUDP

Change-Id: I69f24887601e491d6d722bfeb2952d927df8ad80
Reviewed-on: https://go-review.googlesource.com/4351Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/udpsock_plan9.go

@@ -101,9 +101,11 @@ func (c *UDPConn) WriteTo(b []byte, addr Addr) (int, error) {
 	return c.WriteToUDP(b, a)
 }
 
-// WriteMsgUDP writes a packet to addr via c, copying the payload from
-// b and the associated out-of-band data from oob.  It returns the
-// number of payload and out-of-band bytes written.
+// WriteMsgUDP writes a packet to addr via c if c isn't connected, or
+// to c's remote destination address if c is connected (in which case
+// addr must be nil).  The payload is copied from b and the associated
+// out-of-band data is copied from oob.  It returns the number of
+// payload and out-of-band bytes written.
 func (c *UDPConn) WriteMsgUDP(b, oob []byte, addr *UDPAddr) (n, oobn int, err error) {
 	return 0, 0, syscall.EPLAN9
 }