[DOC] add example to computed properties set() in API docs

Before:
![ember_js_-_ember_computedproperty](https://cloud.githubusercontent.com/assets/514063/4126759/58cc62e8-32ef-11e4-953f-4ca7bf741f08.png)

After:
![ember_js_-_ember_computedproperty](https://cloud.githubusercontent.com/assets/514063/4126755/50cef15a-32ef-11e4-998f-a84a687500bb.png)