net: improve ExampleUDPConn_WriteTo docs

Also updates comment on isConnected field of netFD for clarification.

Change-Id: Icb1b0332e3b4c7802eae00ddc26cd5ba54c82dc2
Reviewed-on: https://go-review.googlesource.com/120955
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/example_test.go

@@ -121,7 +121,8 @@ func ExampleIPv4Mask() {
 }
 
 func ExampleUDPConn_WriteTo() {
-	// Create connection in non-pre-connected state
+	// Unlike Dial, ListenPacket creates a connection without any
+	// association with peers.
 	conn, err := net.ListenPacket("udp", ":0")
 	if err != nil {
 		log.Fatal(err)
@@ -133,7 +134,7 @@ func ExampleUDPConn_WriteTo() {
 		log.Fatal(err)
 	}
 
-	// Write data to the desired address
+	// The connection can write data to the desired address.
 	_, err = conn.WriteTo([]byte("data"), dst)
 	if err != nil {
 		log.Fatal(err)

src/net/fd_unix.go

@@ -22,7 +22,7 @@ type netFD struct {
 	// immutable until Close
 	family      int
 	sotype      int
-	isConnected bool
+	isConnected bool // handshake completed or use of association with peer
 	net         string
 	laddr       Addr
 	raddr       Addr

src/net/fd_windows.go

@@ -32,7 +32,7 @@ type netFD struct {
 	// immutable until Close
 	family      int
 	sotype      int
-	isConnected bool
+	isConnected bool // handshake completed or use of association with peer
 	net         string
 	laddr       Addr
 	raddr       Addr

src/net/net_fake.go

@@ -50,7 +50,7 @@ type netFD struct {
 
 	// unused
 	pfd         poll.FD
-	isConnected bool
+	isConnected bool // handshake completed or use of association with peer
 }
 
 // socket returns a network file descriptor that is ready for