Well, unless the MDN team works differently nowadays,
dev-doc-needed indicates that the related browser compatibility data entry needs to be updated once this feature landed.
I also assume that this will initially be implemented behind a pref. In that case it should also be documented at https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Experimental_features. So I added the keyword back.
Once there's a bug to ship
@property (maybe together with the DOM API),
dev-doc-needed should also be added there in order to update the browser compat data and remove it from the experimental features page again once that bug is closed.