net: document ReadMsgUnix, WriteMsgUnix

Fixes #3247.

R=golang-dev, jsing
CC=golang-dev
https://golang.org/cl/5784051

src/pkg/net/unixsock_posix.go

@@ -208,8 +208,8 @@ func (c *UnixConn) SetWriteBuffer(bytes int) error {
 }
 
 // ReadFromUnix reads a packet from c, copying the payload into b.
-// It returns the number of bytes copied into b and the return address
-// that was on the packet.
+// It returns the number of bytes copied into b and the source address
+// of the packet.
 //
 // ReadFromUnix can be made to time out and return
 // an error with Timeout() == true after a fixed time limit;
@@ -264,6 +264,11 @@ func (c *UnixConn) WriteTo(b []byte, addr Addr) (n int, err error) {
 	return c.WriteToUnix(b, a)
 }
 
+// ReadMsgUnix reads a packet from c, copying the payload into b
+// and the associated out-of-band data into oob.
+// It returns the number of bytes copied into b, the number of
+// bytes copied into oob, the flags that were set on the packet,
+// and the source address of the packet.
 func (c *UnixConn) ReadMsgUnix(b, oob []byte) (n, oobn, flags int, addr *UnixAddr, err error) {
 	if !c.ok() {
 		return 0, 0, 0, nil, syscall.EINVAL
@@ -276,6 +281,9 @@ func (c *UnixConn) ReadMsgUnix(b, oob []byte) (n, oobn, flags int, addr *UnixAdd
 	return
 }
 
+// WriteMsgUnix 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.
 func (c *UnixConn) WriteMsgUnix(b, oob []byte, addr *UnixAddr) (n, oobn int, err error) {
 	if !c.ok() {
 		return 0, 0, syscall.EINVAL