net/http: document CONNECT more

Fixes #22554

Change-Id: I624f2883489a46d7162c11f489c2f0a0ec5a836f
Reviewed-on: https://go-review.googlesource.com/86277
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/request.go

@@ -98,6 +98,10 @@ var reqWriteExcludeHeader = map[string]bool{
 type Request struct {
 	// Method specifies the HTTP method (GET, POST, PUT, etc.).
 	// For client requests an empty string means GET.
+	//
+	// Go's HTTP client does not support sending a request with
+	// the CONNECT method. See the documentation on Transport for
+	// details.
 	Method string
 
 	// URL specifies either the URI being requested (for server

src/net/http/transport.go

@@ -73,6 +73,15 @@ const DefaultMaxIdleConnsPerHost = 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.
+//
+// The Transport will send CONNECT requests to a proxy for its own use
+// when processing HTTPS requests, but Transport should generally not
+// be used to send a CONNECT request. That is, the Request passed to
+// the RoundTrip method should not have a Method of "CONNECT", as Go's
+// HTTP/1.x implementation does not support full-duplex request bodies
+// being written while the response body is streamed. Go's HTTP/2
+// implementation does support full duplex, but many CONNECT proxies speak
+// HTTP/1.x.
 type Transport struct {
 	idleMu     sync.Mutex
 	wantIdle   bool                                // user has requested to close all idle conns