Describe event handling for dead letter sink routing

Some channel implementations do enhance the CloudEvents in the case they are forwarding them to the configured Dead Letter Sink. Details can be read here: https://knative.dev/docs/eventing/event-delivery/#configuring-channel-event-delivery

- The behavior is not just interesting for Channel implementations, it is something to be good for broker implementations as well.

The Knative Eventing Spec are currently not mentioning the three different extension attributes (`knativeerrordest`, `knativeerrorcode` and `knativeerrordata`), this should be fixed

Read the linked Knative Eventing event-delivery documentation and inspect the Eventing Spec sections covering dead letter sink routing. Document the three extension attributes—knativeerrordest, knativeerrorcode, and knativeerrordata—and clarify that the behavior applies to both Channel and broker implementations.

Written by the indexing model from the issue text.