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