but the class definition only defines
What am I missing here?
Hi. It looks like this comment is out of date for a while now. There is no EventsFields as you noted - the event handling mechanism migrated to the EventManager class quite a while ago. This comment should definitely be fixed (and will be shortly).
If you wish to follow this further, the method Object>>actionMap is how the appropriate EventManager instance is found. Further, the related methods are in the *System-Object Events-accessing method category (and sub-categories).