update doc to make it more clear

Author: jichenjc

docs/book/src/clusteropenstack/configuration.md

@@ -101,7 +101,7 @@ The following variables are set.
 | OPENSTACK_CLOUD | The cloud name which is used as second argument |
 | OPENSTACK_CLOUD_YAML_B64 | The secret used by Cluster API Provider OpenStack accessing OpenStack |
 | OPENSTACK_CLOUD_PROVIDER_CONF_B64 | The content of [cloud.conf](https://git.k8s.io/cloud-provider-openstack/docs/openstack-cloud-controller-manager/using-openstack-cloud-controller-manager.md#deploy-a-kubernetes-cluster-with-openstack-cloud-controller-manager-using-kubeadm) which is used by OpenStack cloud provider |
-| OPENSTACK_CLOUD_CACERT_B64 | (Optional) The content of your custom CA file which can be specified in your clouds.yaml by `ca-file` |
+| OPENSTACK_CLOUD_CACERT_B64 | The content of your custom CA file which can be specified in your clouds.yaml by `ca-file`, mandatory when openstack endpoint is `https` |
 
 Note: Only the [external cloud provider](https://cluster-api-openstack.sigs.k8s.io/topics/external-cloud-provider.html) supports [Application Credentials](https://docs.openstack.org/keystone/latest/user/application_credentials.html).