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