Document how to customize forms in ApplicationEditor

Summary:
Ref T9132. I think the featureset is approximatley stable, so here's some documentation.

I also cleaned up a handful of things in the UI and tried to make them more obvious or more consistent.

Test Plan: Read documentation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14718