OAuthServer - harden things up a bit

Summary: This is the hardening work mentioned in T887#86529. Also take a documentation pass for accuracy about these changes and formatting. Ref T4593.

Test Plan: unit tests...! generated diviner docs and oauthserver doc looked good

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T4593

Differential Revision: https://secure.phabricator.com/D11298