[DOC beta] Document the didTransition and loading action events

This is a fix for https://github.com/emberjs/website/issues/1508

![screen shot 2014-05-09 at 11 20 28 am](https://cloud.githubusercontent.com/assets/54056/2929786/7e7c0934-d78d-11e3-9f5a-40bb6dc6179c.png)