should the object model guide talk about factory registration?

I'd like to report a shortcoming I've noticed in the guide, and suggest adding a further-reading link between two of its pages. I don't understand the issue well enough to suggest another edit.

While adding the router service to an event-handler object (EmberObject subclass) I got the error `Attempting to lookup an injected property on an object without a container, ensure that the object was instantiated via a container`, which seems similar to https://stackoverflow.com/questions/34970881/inject-a-service-into-an-ember-object-not-an-ember-controller

Everything else on the event handler has been working ok with the `import` and `create` approach, but when I tried to load in the router service I hit the error.
The assert is useful, and has helped me figure out what to do next, but it seems like the guide should cover this. Instead it only talks about using `create`.

Explaining why this is needed or different from `import`/`create` would be great (maybe transcribing @locks's stackoverflow post), but it'd be simpler to put something short at the bottom of [classes and instances](https://guides.emberjs.com/release/object-model/classes-and-instances/) suggesting further reading of [dependency injection](https://guides.emberjs.com/release/applications/dependency-injection/) or even [registering singletons vs non-singletons](https://guides.emberjs.com/release/applications/dependency-injection/#toc_registering-singletons-vs-non-singletons) as further reading.