Adds Region events documentation.

Resolves #1771.

These event docs are not so good right now, but it's the best we can do at the moment, 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. I think we communicate that same information by simply including the events in the list.

Instead of examples, I think 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. Perhaps a little paragraph at the start of each `events` section will work to accomplish this.
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. An example of that is how I included whether or not the view is attached to the `document` at the time of the show events.

Extending the API format to support that is beyond the scope of this PR, though, so that shtuff can come later.
Why is this here?