improve AbstractActor, #21717

* Receive class that wraps PartialFunction, to avoid
  scary scala types
* move AbstractActorContext to AbstractActor.ActorContext
* converting docs, many, many UntypedActor
* removing UntypedActor docs
* add unit test for ReceiveBuilder
* MiMa filters
* consistent use of getContext(), self(), sender()
* rename cross references
* migration guide
* skip samples for now
* improve match type safetyi, add matchUnchecked
  * the `? extends P` caused code like this to compile:
    `match(String.class, (Integer i) -> {})`
  * added matchUnchecked, since it can still be useful (um, convenient)
    to be able to do:
    `matchUnchecked(List.class, (List<String> list) -> {})`
* eleminate some scala.Option
  * preRestart
  * findChild
  * ActorIdentity.getActorRef

akka-docs/rst/java/code/docs/actorlambda/MyBoundedActor.java

@@ -0,0 +1,14 @@
+/**
+ * Copyright (C) 2009-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+
+package docs.actorlambda;
+
+//#my-bounded-untyped-actor
+import akka.dispatch.BoundedMessageQueueSemantics;
+import akka.dispatch.RequiresMessageQueue;
+
+public class MyBoundedActor extends MyActor 
+  implements RequiresMessageQueue<BoundedMessageQueueSemantics> {
+}
+//#my-bounded-untyped-actor