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