net/http: clarify docs on Request.Proto field

No need to say "by default" because there is no alternative and no way
to override. Always HTTP/2.0 is officially spelled HTTP/2 these days.

Fixes #13985 harder

Change-Id: Ib1ec03cec171ca865342b8e7452cd4c707d7b770
Reviewed-on: https://go-review.googlesource.com/18720Reviewed-by: Rob Pike <r@golang.org>

src/net/http/request.go

@@ -93,8 +93,8 @@ type Request struct {
 	// The protocol version for incoming server requests.
 	//
 	// For client requests these fields are ignored. The HTTP
-	// transport code uses either HTTP/1.1 or HTTP/2.0 by default,
-	// depending on what the server supports.
+	// client code always uses either HTTP/1.1 or HTTP/2.
+	// See the docs on Transport for details.
 	Proto      string // "HTTP/1.0"
 	ProtoMajor int    // 1
 	ProtoMinor int    // 0

src/net/http/transport.go

@@ -57,6 +57,10 @@ const DefaultMaxIdleConnsPerHost = 2
 //
 // A Transport is a low-level primitive for making HTTP and HTTPS requests.
 // 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.
 type Transport struct {
 	idleMu     sync.Mutex
 	wantIdle   bool // user has requested to close all idle conns