Clarify that `$event` is a jQuery Event Object

I couldn't find documentation on this, but it looks like the `$event`, e.g., inside an ngClick directive, is an instance of a jQuery Event object. Is this correct? If so, then I think this is important information to share in the documentation. Otherwise people who are relatively new to this framework will have no idea how to use the objects that are passed around without finding crafty ways to run test code and inspect objects.