[DOC Beta] Document the route's controller property

This pr adds API docs for Ember.Route#controller as mentioned in https://github.com/emberjs/website/issues/1509

![screen shot 2014-05-08 at 7 45 42 pm](https://cloud.githubusercontent.com/assets/54056/2923012/16a22aa0-d70b-11e3-92ce-e134714222b6.png)