Fix many lies in the "User Roles" document
Summary: Fixes T3047. Update this document and remove some lies ("menu bar is read in admin interfaces"!!!!).
Test Plan:
- Read text.
- Searched for "System Agent" in the UI and replaced it with "bot" or "bot/script" or similar.
Reviewers: chad, btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T3047
Differential Revision: https://secure.phabricator.com/D8675