Update param types for addRegion method

@thejameskyle LMK if this is correct
So if we think about this in terms of Javascript, and not 'Classes,' what w really accept is any method that returns an instance of a Region. There's no difference between a constructor and:

```js
var lalala = function() {
   return new Marionette.Region();
}
```

Because of this, what we really want is something like:

```js
@param Marionette.Region
@param Function @returns Marionette.Region
```

I just made up that syntax there, but I hope it communicates the idea.