net/http: change TimeoutHandler's docs to match its new interfaces

As of Go 1.13rc1, TimeoutHandler supports the Flusher and Pusher interfaces and
this change corrects its documentation to say that.

Fixes #33769
Updates #29193

Author: jmhodges

src/net/http/server.go

@@ -3177,8 +3177,8 @@ func (srv *Server) onceSetNextProtoDefaults() {
 // After such a timeout, writes by h to its ResponseWriter will return
 // ErrHandlerTimeout.
 //
-// TimeoutHandler buffers all Handler writes to memory and does not
-// support the Hijacker or Flusher interfaces.
+// TimeoutHandler supports the Flusher and Pusher interfaces but does not
+// support the Hijacker interface.
 func TimeoutHandler(h Handler, dt time.Duration, msg string) Handler {
 	return &timeoutHandler{
 		handler: h,