deprecate closure-taking Props factories, see #3081

- base Props on Deploy, Class and Seq[Any] (i.e. constructor args)
- remove deprecated Props usage from akka-docs sample code
- rewrite UntypedActorDocTestBase
- rewrite Java/Scala doc section on actor creation
- add migration guide entry

akka-docs/rst/scala/code/docs/testkit/TestkitDocSpec.scala

@@ -208,9 +208,7 @@ class TestkitDocSpec extends AkkaSpec with DefaultTimeout with ImplicitSender {
 
   "demonstrate probe watch" in {
     import akka.testkit.TestProbe
-    val target = system.actorOf(Props(new Actor {
-      def receive = Actor.emptyBehavior
-    }))
+    val target = system.actorOf(Props.empty)
     //#test-probe-watch
     val probe = TestProbe()
     probe watch target
@@ -237,7 +235,7 @@ class TestkitDocSpec extends AkkaSpec with DefaultTimeout with ImplicitSender {
     import akka.actor.Props
     //#test-probe-forward
     val probe = TestProbe()
-    val source = system.actorOf(Props(new Source(probe.ref)))
+    val source = system.actorOf(Props(classOf[Source], probe.ref))
     val dest = system.actorOf(Props[Destination])
     source ! "start"
     probe.expectMsg("work")