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

Closes #1151.

The following is an incomplete draft. However, I wanted to get comments immediately, in any case anything was severely wrong.

The doc should be updated to include:

- [ ] How context-local objects in Flask are implemented
My understanding is that context-locals are "pretty much" thread-locals, but are distinct in that they bind to the current thread *or* greenlet. Is that incorrect?