Maniphest - refine maniphest.statuses documentation slightly

Summary: Fixes T7164. Adds some details about how the statuses will show up in the UI.

Test Plan: Read the text

Reviewers: epriestley, chad

Reviewed By: chad

Subscribers: Korvin, epriestley

Maniphest Tasks: T7164

Differential Revision: https://secure.phabricator.com/D11686