Correction needed for Defining a Route's Model - default model hook behavior

This is the section that isn't quite right, identified by runspired:

```
If you do not define a model hook for a route, it will default to using Ember Data to look up the record, as shown below:

model(params) {
return this.store.find('post', params.post_id);
}
```

A more accurate statement is:

> If you do not define a model hook for a route, it will default to using Ember's DefaultStore interface to look up the record, as shown below:

So any data-related plugin has to address this behavior. It's not unique to Ember Data.

To-do:
- [ ] figure out how to explain this in a way that doesn't require us to also explain the undocumented DefaultStore, and makes it clear that any data plugin will result in some behavior like this
- [ ] replace the incorrect explanation currently published in the Guides
- [ ] Make the PR to the `master` branch of this repository

Background info, which should not go in the Guides:

The DefaultStore is undocumented, but lives here: https://github.com/emberjs/ember.js/blob/52ddee883414f655511ef71d6f875f3fc16028e3/packages/%40ember/-internals/routing/lib/system/route.ts#L2145-L2176

the best mention in the docs is this one: https://api.emberjs.com/ember/3.9/classes/Route/properties/store?anchor=routeName&show=inherited%2Cprivate

what actually happens is that libraries like ember-model and ember-data "Do A Bad Thing"™ because of this DefaultStore. It forces us to eagerly inject store onto all routes to replace this store property and it forces us Ember Data to support store.find.