Treat worldwide as restofworld

https://bugzilla.mozilla.org/show_bug.cgi?id=940561

Handle the "worldwide" slug as if it were "restofworld" instead. I also fixed a bug where region detail would [500](https://marketplace.firefox.com/api/v1/services/region/foo/) instead of 404 and corrected the app update docs to show that you cannot change the regions through the API.