Update account roles documentation and remove actAsUser

Summary:
Ref T8387. This describes changes I haven't made yet, but plan to make.

Also removes the long-deprecated actAsUser capability so I can remove the caveat about it from the documentation.

Test Plan: grep, reading

Reviewers: btrahan, eadler

Reviewed By: btrahan, eadler

Subscribers: eadler, epriestley

Maniphest Tasks: T8387

Differential Revision: https://secure.phabricator.com/D13120