clarify context needed for ActorDSL.actor() and add @implicitNotFound

akka-actor-tests/src/test/scala/akka/actor/ActorDSLSpec.scala

@@ -7,14 +7,21 @@ package akka.actor
 import language.postfixOps
 
 import akka.testkit.{ AkkaSpec, EventFilter }
-//#import
 import akka.actor.ActorDSL._
-//#import
 import akka.event.Logging.Warning
 import scala.concurrent.{ Await, Future }
 import scala.concurrent.duration._
 import java.util.concurrent.TimeoutException
 
+class ActorDSLDummy {
+  //#import
+  import akka.actor.ActorDSL._
+  import akka.actor.ActorSystem
+
+  implicit val system = ActorSystem("demo")
+  //#import
+}
+
 class ActorDSLSpec extends AkkaSpec {
 
   val echo = system.actorOf(Props(new Actor {

akka-actor/src/main/scala/akka/actor/ActorRefProvider.scala

@@ -13,6 +13,7 @@ import akka.util.Collections.EmptyImmutableSeq
 import scala.util.{ Success, Failure }
 import java.util.concurrent.atomic.AtomicLong
 import scala.concurrent.{ ExecutionContext, Future, Promise }
+import scala.annotation.implicitNotFound
 
 /**
  * Interface for all ActorRef providers to implement.
@@ -156,6 +157,7 @@ trait ActorRefProvider {
  * Interface implemented by ActorSystem and ActorContext, the only two places
  * from which you can get fresh actors.
  */
+@implicitNotFound("implicit ActorRefFactory required: if outside of an Actor you need an implicit ActorSystem, inside of an actor this should be the implicit ActorContext")
 trait ActorRefFactory {
   /**
    * INTERNAL API

akka-docs/rst/scala/actors.rst

@@ -162,8 +162,9 @@ infrastructure is bundled in the following import:
 
 .. includecode:: ../../../akka-actor-tests/src/test/scala/akka/actor/ActorDSLSpec.scala#import
 
-This import is assumed for all code samples throughout this section. To define
-a simple actor, the following is sufficient:
+This import is assumed for all code samples throughout this section. The
+implicit actor system serves as :class:`ActorRefFactory` for all examples
+below. To define a simple actor, the following is sufficient:
 
 .. includecode:: ../../../akka-actor-tests/src/test/scala/akka/actor/ActorDSLSpec.scala#simple-actor