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 this 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 effectively 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.
I took it from the docs [we already had merged in.](https://github.com/marionettejs/backbone.marionette/blob/master/api/region-manager.jsdoc#L255-L256) If we want to change it we should do it across all of the docs.