Hi!
Responding to your questions much later, but the process hasn't changed that much since you originally posted. The pipeline steps reference page is generated automatically; it should update frequently and there are constant improvements out there for that doc generation right now. If your plugin is relatively new, then it might not have been picked up yet (though at this point depending, it should be). I might need to check on that job to make sure it's still going. It's INCREDIBLY resource intensive (basically it creates a Jenkins that knows about every single plugin in the Jenkins update center) (you might not want to try that in your own Jenkins environment!), and I need to see how it's doing on some of the new processes.
You don't need to do anything different with your plugin or add it to a list. Recently, there was an update to make the documentation more complete. So if your step integrated with the workflow-scm plugin (like it was a step in that you called it from "step" or "wrap") it'll be added. That should get rolled in soon, and the page will be updated shortly after.
Hope this answers your questions!