R1:6eb91b2a0e79

Improve documentation for System Agents and other account roles

Summary: Explain this stuff better and add some documentation links.

Test Plan: Read documentation, viewed account edit interfaces.

Reviewers: btrahan, vrana, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T834

Differential Revision: https://secure.phabricator.com/D2158