ZeroMQ module's ConcurrentSocketActorSpec fails.

* Fixed usage of extension. See #1746
* Clarified usage of load extensions in docs. See #1745

akka-docs/java/extending-akka.rst

@@ -54,6 +54,12 @@ Loading from Configuration
 To be able to load extensions from your Akka configuration you must add FQCNs of implementations of either ``ExtensionId`` or ``ExtensionIdProvider``
 in the "akka.extensions" section of the config you provide to your ``ActorSystem``.
 
+::
+
+    akka {
+      extensions = ["akka.docs.extension.ExtensionDocTestBase.CountExtension"]
+    }
+
 Applicability
 =============