Modernize CustomField documentation
Summary: Fixes T4102. Document all the new stuff that CustomField supports now, and all the applications you can use it with.
Test Plan: Generated and read documentation.
Reviewers: chad, btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T4102
Differential Revision: https://secure.phabricator.com/D8541