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:

- [ ] How context-local objects in Flask are implemented

Will squash commits later.
I think the last sentence is actually contradicting the previous story about why ``threading.local()`` is not an option, since it imploes that "specific to greenlet/thread" == "context-specific"