net: add example for net.UDPConn.WriteTo function

The current documentation of the WriteTo function is very poor and it
is difficult to deduce how to use it correctly. A good example will
make things much easier.

Fixes #25456

Change-Id: Ibf0c0e153afae8f3e0d7d765d0dc9bcbfd69bfb1
Reviewed-on: https://go-review.googlesource.com/113775
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/example_test.go

@@ -119,3 +119,23 @@ func ExampleIPv4Mask() {
 	// Output:
 	// ffffff00
 }
+
+func ExampleUDPConn_WriteTo() {
+	// Create connection in non-pre-connected state
+	conn, err := net.ListenPacket("udp", ":0")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer conn.Close()
+
+	dst, err := net.ResolveIPAddr("udp", "192.0.2.1:2000")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Write data to the desired address
+	_, err = conn.WriteTo([]byte("data"), dst)
+	if err != nil {
+		log.Fatal(err)
+	}
+}