Links to Region docs in Application docs.

Resolves #1366
I am :-1: to linking to the RegionManager docs up here. I'd rather we all but pretend it doesn't exist so that people begin thinking about it as an API mixin more than a Class. I think it's enough to simply mention the function – the entire next few sections is about the usage of the function.

One of my issues with linking to the RegionManager docs is that the API between RegionManager and Application doesn't line up 1-to-1. You really need to look at the source code (or read the App's docs) to understand how the RegionManager API works from an Application. It's lame, but unfortunately true :(

> we can also add regions via the regions: {} option eh, we should note that

Separate issue, I think. This is mostly about adding links to the already-existing docs. File an issue and we will fix in another PR?