[DOC beta] Update the FEATURES.isEnabled docs so it doesn't look like a ...

...method on the Ember object.

Currently the docs make it look like the `isEnabled` method is attached to the Ember object.
![screen shot 2014-04-11 at 10 22 25 am](https://cloud.githubusercontent.com/assets/54056/2681074/3ade6218-c189-11e3-8f74-87d6971fec72.png)

This pr updates the docs to make it clear that the `isEnabled` method is attached to the `Ember.FEATURES` object.
![screen shot 2014-04-11 at 10 21 52 am](https://cloud.githubusercontent.com/assets/54056/2681080/49d77778-c189-11e3-922e-03e80624d18b.png)