R1:c15501fc9b06

Add clearer language and options to Arcanist install guides

Summary: Fixes T12315. Reworks the copy placement a little, adds more links.

Test Plan: Read in sandbox

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12315

Differential Revision: https://secure.phabricator.com/D17413