Add Home menu default Dashboard documentation

Summary: From Z1336, we don't currently document anywhere how the default dashboard works. I should also update the copy in the UI. Ref T12969

Test Plan: regenerate docs, read carefully

Reviewers: epriestley

Reviewed By: epriestley

Spies: Korvin

Maniphest Tasks: T12969

Differential Revision: https://secure.phabricator.com/D18454