Improve Arcanist event doucmentation

Summary:

• Soft dependencies on D3423, D3422, D3421.
• Document the availability of 'workflow' on all events.
• Improve instructions on building, configuring and debugging event handlers.

Test Plan: Read documentation.

Reviewers: vrana, nh

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3424