net/http: update Transport doc example to not disable http2

The old Transport example ended up disabling HTTP/2.

Use a better example.

Fixes #17051
Fixes #17296

Change-Id: I6feca168744131916e8bf56c829b4d4b50e304ee
Reviewed-on: https://go-review.googlesource.com/33094Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/doc.go

@@ -44,7 +44,8 @@ For control over proxies, TLS configuration, keep-alives,
 compression, and other settings, create a Transport:
 
 	tr := &http.Transport{
-		TLSClientConfig:    &tls.Config{RootCAs: pool},
+		MaxIdleConns:       10,
+		IdleConnTimeout:    30 * time.Second,
 		DisableCompression: true,
 	}
 	client := &http.Client{Transport: tr}

src/net/http/transport.go

@@ -68,8 +68,10 @@ const DefaultMaxIdleConnsPerHost = 2
 // For high-level functionality, such as cookies and redirects, see Client.
 //
 // Transport uses HTTP/1.1 for HTTP URLs and either HTTP/1.1 or HTTP/2
-// for HTTPS URLs, depending on whether the server supports HTTP/2.
-// See the package docs for more about HTTP/2.
+// for HTTPS URLs, depending on whether the server supports HTTP/2,
+// and how the Transport is configured. The DefaultTransport supports HTTP/2.
+// To explicitly enable HTTP/2 on a transport, use golang.org/x/net/http2
+// and call ConfigureTransport. See the package docs for more about HTTP/2.
 type Transport struct {
 	idleMu     sync.Mutex
 	wantIdle   bool                                // user has requested to close all idle conns
@@ -113,7 +115,9 @@ type Transport struct {
 	DialTLS func(network, addr string) (net.Conn, error)
 
 	// TLSClientConfig specifies the TLS configuration to use with
-	// tls.Client. If nil, the default configuration is used.
+	// tls.Client.
+	// If nil, the default configuration is used.
+	// If non-nil, HTTP/2 support may not be enabled by default.
 	TLSClientConfig *tls.Config
 
 	// TLSHandshakeTimeout specifies the maximum amount of time waiting to
@@ -172,7 +176,8 @@ type Transport struct {
 	// called with the request's authority (such as "example.com"
 	// or "example.com:1234") and the TLS connection. The function
 	// must return a RoundTripper that then handles the request.
-	// If TLSNextProto is nil, HTTP/2 support is enabled automatically.
+	// If TLSNextProto is not nil, HTTP/2 support is not enabled
+	// automatically.
 	TLSNextProto map[string]func(authority string, c *tls.Conn) RoundTripper
 
 	// ProxyConnectHeader optionally specifies headers to send to