In Conduit, let checkbox constraints self-document

Summary:
Ref T13195. Ref PHI851. Currently, checkbox constraints don't tell you what values are supported on the API page.

You can figure this out with "Inspect Element" or by viewing the source code, but just render a nice table instead.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13195

Differential Revision: https://secure.phabricator.com/D19641