Am I just plain stupid, or documentation for it is not helpful at all?
How am I supposed to guess which arguments the handler is supposed to have?
Almost every events in the Unity documentation lack this information
The easiest way to find out is to type in your IDE, and use the “Go to definition” feature.
The signature of the event is the following:
public static event UnityAction<Scene, Scene> activeSceneChanged;
Otherwise, Unity recently opened a Github with the source code of the engine. You will be able to find the event definition here