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 have the same intuition. This page may be better titled "Runtime globals", and should avoid discussing context locals except in the sense that these global objects are unique to a specific runtime context since they are mostly implementation details.