Adds Region events documentation.

Resolves #1771.

These event docs are junk right now, but it's the best we can do for now, I think.

Presently, we give an example **for every single event**, which is just kind of unnecessary, I think. Rendering a view 6 times and registering a different callback tells the user **nothing**, other than the fact that, hey, you can register a callback for an event.

Instead, what we need are:

1. A clear, simple way to communicate to people that each of these points describe a *pair* of things: an event and its callback.
2. The list of arguments that are passed to the callbacks.
3. A concise definition telling people when it is triggered. We can use foresight to add additional information into this description to handle the most-likely use cases, such as my inclusion of whether or not the view is attached to the `document` at the time of the show events.