No explanation how to use visual editor with singletons

Hi,

I've followed the documentation on setting up the visual editor, but couldn't find anything on how to use it with a singleton collection. I've spent about 30 minutes on trial & error until I figured out that I need to put `1` for `item`.

Usual first instinct here would probably be to just not provide anything for `item`, but that doesn't work. And both Discussions & Issues on the Directus Repo and the Directus Community all seem to not have anything about this. Due to all of this, I think it's especially important to mention this somewhere on the docs.