SortableMixin: reference to arrangedContent in top example and adding the actual property to the API docs.

At the moment, the [docs on SortableMixin](http://emberjs.com/api/classes/Ember.SortableMixin.html) don't mention `arrangedContent` at all. I added it to the examples at the top, but also added documentation on the actual property so that it will be included in the "Properties" tab of the docs in the future.