Subscribe model description missing and model scubscription QoL

[ahilloffis]

Subscribing to a model change is a nice feature but it's missing documentation and the usage is a little counterintuitive:

self.context.subscribe_model(self, type(model))

Instead of type(model) as a parameter, I would expect model or MyClass.

Also, instead of just subscribing to the model and handling the change in the def on_change_model(self, model) method of the class, passing a method, like in other context subscriptions, would be a nice feature. If needed, I will create another issue for this topic.