Fix confusing documentation on maniphest.default-priority

Summary:
Fixes T6056. This documentation is out of date and not very useful.

We could probably fold this option into maniphest.priorities at some point.

Test Plan: Read documentation, clicked link.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6056

Differential Revision: https://secure.phabricator.com/D10450