x/crypto/ssh: remove misleading comment, add example

Add an example for using the PublicKeys AuthMethod.

Change-Id: I3fe02bb3c9b8ccf313d72858328c8576cbf3eb06
Reviewed-on: https://go-review.googlesource.com/22250
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/example_test.go

@@ -113,8 +113,7 @@ func ExampleNewServerConn() {
 }
 
 func ExampleDial() {
-	// An SSH client is represented with a ClientConn. Currently only
-	// the "password" authentication method is supported.
+	// An SSH client is represented with a ClientConn.
 	//
 	// To authenticate with the remote server you must pass at least one
 	// implementation of AuthMethod via the Auth field in ClientConfig.
@@ -147,6 +146,39 @@ func ExampleDial() {
 	fmt.Println(b.String())
 }
 
+func ExamplePublicKeys() {
+	// A public key may be used to authenticate against the remote
+	// server by using an unencrypted PEM-encoded private key file.
+	//
+	// If you have an encrypted private key, the crypto/x509 package
+	// can be used to decrypt it.
+	key, err := ioutil.ReadFile("/home/user/.ssh/id_rsa")
+	if err != nil {
+		log.Fatalf("unable to read private key: %v", err)
+	}
+
+	// Create the Signer for this private key.
+	signer, err := ssh.ParsePrivateKey(key)
+	if err != nil {
+		log.Fatalf("unable to parse private key: %v", err)
+	}
+
+	config := &ssh.ClientConfig{
+		User: "user",
+		Auth: []ssh.AuthMethod{
+			// Use the PublicKeys method for remote authentication.
+			ssh.PublicKeys(signer),
+		},
+	}
+
+	// Connect to the remote server and perform the SSH handshake.
+	client, err := ssh.Dial("tcp", "host.com:22", config)
+	if err != nil {
+		log.Fatalf("unable to connect: %v", err)
+	}
+	defer client.Close()
+}
+
 func ExampleClient_Listen() {
 	config := &ssh.ClientConfig{
 		User: "username",