5.5: Document the new event classes

[TimWolla]

See WoltLab/WCF#4000 and WoltLab/WCF#4265.

• Add to migration docs.
• Incorporate Update event handling for IEvents WCF#4301
• Update the Event (Listener) section and examples.

[TimWolla]

These are added to the migration docs, but the event section will also need to be updated.

[TimWolla]

Moving this to 6.0, because WoltLab/WCF#5060 comes in helpful.

[TimWolla]

Or rather reopening.