Add controller model property to documentation.

This is just a small documentation addition to ensure that the `model` property of the `ControllerMixin` shows up in the API docs. It also warns against using the `content` property directly, since the documentation for `ControllerContentModelAliasDeprecation ` states:

> This change reduces many caveats with model/content, and also sets a simple ground rule: Never set a controllers content, rather always set it's model and ember will do the right thing.

There's already quite a bit of confusion about whether to use `model` or `content`, and I think it would help if `model` was publicly documented.