Clarify the behavior of "audit.can-author-close-audit"

Summary:
Ref T13631. This option has a behavior other than the behavior implied by the name and documented.

Document the correct behavior, at least. This can likely be removed after T10574.

Test Plan: Read config option help in Config.

Maniphest Tasks: T13631

Differential Revision: https://secure.phabricator.com/D21610