Document all the hypothetical URI features we plan to support soon

Summary: Ref T10748. Ref T10366. This documents how everything is planned to work shortly.

Test Plan: Read documentation.

Reviewers: chad

Reviewed By: chad

Subscribers: eadler, scode

Maniphest Tasks: T10366, T10748

Differential Revision: https://secure.phabricator.com/D15817