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.
"to determine what".

Also i think the footnotes are mangled, you reference ``[2]`` twice.