CRD: Document cleanup on delete. (#6075)

Make it clear that deleting a CRD deletes everything in it.

docs/tasks/access-kubernetes-api/extend-api-custom-resource-definitions.md

@@ -151,6 +151,23 @@ metadata:
   resourceVersion: ""
   selfLink: ""
 ```
+
+## Delete a CustomResourceDefinition
+
+When you delete a CustomResourceDefinition, the server will uninstall the RESTful API  endpoint
+and **delete all custom objects stored in it**.
+
+```shell
+kubectl delete -f resourcedefinition.yaml
+kubectl get crontabs
+```
+
+```console
+Error from server (NotFound): Unable to list "crontabs": the server could not find the requested resource (get crontabs.stable.example.com)
+```
+
+If you later recreate the same CustomResourceDefinition, it will start out empty.
+
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