Transactions - finish making built-in transaction types implementation optional

Summary: Fixes T6403. Remaining built-ins were already built-in effectively so this is a small re-factor plus some docs. I probably wouldn't have written anything if not for the TODO so please feel free to tell me to write something else or what have you.

Test Plan: NA since this didn't actually change anything.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6403

Differential Revision: https://secure.phabricator.com/D12937