In general, behavior documentation can be accessed by clicking the "Help" button in their settings panel. You can add a custom behavior by selecting it from the "Logic & Math" section in the behavior editor (added at the bottom).
The behavior docs are sparse for this one, since it doesn't have any logic by default. Once you add the behavior and open its code editor, there is a "How Does This Work" button at the top, which has a short getting started guide. The code editor itself also has several templates to start from.
I'll be adding a Custom Behaviors section to the behaviors documentation on the site as well, but that's not live yet.