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.
"worker locals" requires a useful definition of what a worker is. 

How about request/view and application locals for context locals available in a request and application context respectively.