Document setupController hook behavior further

The default implementation doesn't update the `model` property of the controller
if the resolved model is undefined.  This isn't currently documented.

packages/ember-routing/lib/system/route.js

@@ -1703,7 +1703,8 @@ let Route = EmberObject.extend(ActionHandler, Evented, {
     model supplied by the `model` hook.
 
     By default, the `setupController` hook sets the `model` property of
-    the controller to the `model`.
+    the controller to the `model`. If `model` is `undefined`, the default
+    implementation will _not_ update the `model` property of the controller.
 
     If you implement the `setupController` hook in your Route, it will
     prevent this default behavior. If you want to preserve that behavior