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.
Yes, this is tough, but I agree. I am hoping to add a "Implementation" section soon, in which case the clearest thing to say at this point is probably "data specific to the current request".