Add a synopsis and example for "arc help land"
Add a synopsis and example for "arc help land"

Summary: Ref T13546. Small documentation fix. Mostly so I can have more things to land.

Test Plan: Ran "arc help land", saw help.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21321

codelabs-ch/arcanist,phacility/arcanist,codelabs-ch/arcanist,phacility/arcanist,phacility/arcanist,Khan/arcanist,codelabs-ch/arcanist,Khan/arcanist,codelabs-ch/arcanist,Khan/arcanist,phacility/arcanist,phacility/arcanist,Khan/arcanist,codelabs-ch/arcanist