eventing: Objects don't have a description via `kubectl explain`

Describe the bug No description when running kubectl explain

$ kubectl explain brokers
KIND:     Broker
VERSION:  eventing.knative.dev/v1beta1

DESCRIPTION:
     <empty>
$ kubectl explain sinkbinding
KIND:     SinkBinding
VERSION:  sources.knative.dev/v1alpha2

DESCRIPTION:
     <empty>

Expected behavior I expected a description like all other k8s objects.

Maybe it comes from the kubectl code and we cannot get explain to work on CRDs.