R1:38403b12beb6

Update Herald documentation for modern policies and beahvior

Summary: Ref T11428. This documentation was a bit misleading and out of date. Update it to reflect modern reality.

Test Plan: Read documentation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11428

Differential Revision: https://secure.phabricator.com/D16384