Update docs for changed regions API

Update docs for #620.

I know that this would cause even more breakage, but I think the signature of [`registerRegion`](http://docs.chaplinjs.org/chaplin.view.html#registerRegion) should be updated, too. This will otherwise be a constant source of confusion. Better to break once cleanly and properly. @paulmillr, I think [your script](https://gist.github.com/paulmillr/5891455) could be updated to handle this change in a similar fashion?

Should be done before #614.