clarify sender of throttle example, #23485

* and use noSender in ActorRefSinkActor

akka-docs/src/main/paradox/scala/project/migration-guide-2.4.x-2.5.x.md

@@ -714,6 +714,8 @@ final ActorRef throttler =
     .run(materializer);
 ```
 
+Note, that when using `Sink.actorRef` the sender of the original message sent to the `thottler` ActorRef will be lost and messages will arrive at the `target` with the sender set to `ActorRef.noSender`. Using this construct it is currently impossible to keep the original sender. Alternatively, you can manually specify a static sender by replacing `Sink.actorRef` with @java[`Sink.foreach(msg -> target.tell(msg, whateverSender))`]@scala[`Sink.foreach(msg => target.tell(msg, whateverSender))`]. You could also calculate a sender by inspecting the `msg`. Be cautious not to use `target.tell(msg, sender())` inside of `Sink.foreach` because the result of `sender()` is undefined or will fail when executed from within a stream.
+
 ## Akka Typed
 
 With the new term @ref:[may change](../common/may-change.md) we will no longer have a different artifact for modules that are not

akka-stream/src/main/scala/akka/stream/impl/ActorRefSinkActor.scala

@@ -32,12 +32,12 @@ import akka.annotation.InternalApi
 
   def receive = {
     case OnNext(elem) ⇒
-      ref ! elem
+      ref.tell(elem, ActorRef.noSender)
     case OnError(cause) ⇒
-      ref ! Status.Failure(cause)
+      ref.tell(Status.Failure(cause), ActorRef.noSender)
       context.stop(self)
     case OnComplete ⇒
-      ref ! onCompleteMessage
+      ref.tell(onCompleteMessage, ActorRef.noSender)
       context.stop(self)
     case Terminated(`ref`) ⇒
       context.stop(self) // will cancel upstream