Explain policy exception rules to users

Summary:
Ref T603. Adds clarifying text which expands on policies and explains exceptions and rules. The goal is to provide an easy way for users to learn about special policy rules, like "task owners can always see a task".

This presentation might be a little aggressive. That's probably OK as we introduce policies, but something a little more tempered might be better down the road.

Test Plan: See screenshot.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7150