document creator requirement, see #2131

akka-docs/java/untyped-actors.rst

@@ -115,6 +115,12 @@ Here is an example:
 This way of creating the Actor is also great for integrating with Dependency Injection
 (DI) frameworks like Guice or Spring.
 
+.. warning::
+
+  You might be tempted at times to offer an ``UntypedActor`` factory which
+  always returns the same instance, e.g. by using a static field. This is not
+  supported, as it works against the meaning of an actor restart, which is
+  described here: :ref:`supervision-restart`.
 
 UntypedActor API
 ================