net/http: make Transport.CancelRequest doc recommend Request.WithContext

The old deprecation docs were referencing another deprecated field.

Fixes #16752

Change-Id: I44a690048e00ddc790a80214ecb7f5bb0a5b7b34
Reviewed-on: https://go-review.googlesource.com/27510Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/net/http/transport.go

@@ -494,8 +494,9 @@ func (t *Transport) CloseIdleConnections() {
 // CancelRequest cancels an in-flight request by closing its connection.
 // CancelRequest should only be called after RoundTrip has returned.
 //
-// Deprecated: Use Request.Cancel instead. CancelRequest cannot cancel
-// HTTP/2 requests.
+// Deprecated: Use Request.WithContext to create a request with a
+// cancelable context instead. CancelRequest cannot cancel HTTP/2
+// requests.
 func (t *Transport) CancelRequest(req *Request) {
 	t.reqMu.Lock()
 	cancel := t.reqCanceler[req]