clarify that v1beta1 CRDs will have to opt-in pruning feature

Author: yue9944882

content/en/docs/tasks/access-kubernetes-api/custom-resources/custom-resource-definitions.md

@@ -874,7 +874,7 @@ OpenAPI v2 Publishing is available as beta since 1.15, and as alpha since 1.14.
 {{< /note >}}
 
 With the OpenAPI v2 Publishing feature enabled, CustomResourceDefinition [OpenAPI v3 validation schemas](#validation) which are [structural](#specifying-a-structural-schema) are published as part
-of the [OpenAPI v2 spec](/docs/concepts/overview/kubernetes-api/#openapi-and-swagger-definitions) from Kubernetes API server.
+of the [OpenAPI v2 spec](/docs/concepts/overview/kubernetes-api/#openapi-and-swagger-definitions) from Kubernetes API server. You need to opt in in v1beta1 CRDs with `spec.preserveUnknownFields: false`. v1 CRDs enable pruning by default.
 
 [kubectl](/docs/reference/kubectl/overview) consumes the published schema to perform client-side validation (`kubectl create` and `kubectl apply`), schema explanation (`kubectl explain`) on custom resources. The published schema can be consumed for other purposes as well, like client generation or documentation.