Rewrite docs for context locals, request contexts, and app contexts

Closes #1151.

The following is an incomplete draft. However, I wanted to request comments immediately, in case anything was severely wrong.

The doc should be updated to include:

- [x] How context-local objects in Flask are implemented

Will squash commits later.
This doesn't explain how Werkzeug's locals solve the problem. It probably should just mention that Werkzeug's locals are also per-greenlet.