Add describtion about TrackingToken parameter for Event Handlers

[smcvb]

It is currently not documented that there is a ParameterResolver in place which returns the TrackingToken in @EventHandler annotated functions which receive their events from a TrackingEventProcessor, whilst it would be beneficial to explain this.
I assume the best location to provide this description is somewhere in part 4, as most users will not require this level of customization.