Clarify Event Subscribers' priority is across all event subscribers, not in the current class

[pbowyer]

The Symfony framework documentation says Ref

In a given subscriber, different methods can listen to the same event. The order in which methods are executed is defined by the priority parameter of each method (the higher the number the earlier the method is called).

I took this to mean the order was specific to the methods in a given subscriber, and was not coordinated per-event across all subscribers. That's not true, and the priority is global in the application.