Improved controllerClass guides

Signed-off-by: Gustavo Carvalho <gustavo.carvalho@container-solutions.com>

docs/guides-controller-class.md

@@ -14,4 +14,6 @@ helm install custom-external-secrets external-secrets/external-secrets --set con
 {% include 'controller-class-store.yaml' %}
 ```
 
-Now, any `ExternalSecret` bound to this secret store will be evaluated by the operator with the controllerClass custom.
+Now, any `ExternalSecret` bound to this secret store will be evaluated by the operator with the controllerClass custom.
+
+> Note: Any SecretStore without `spec.controller` set will be considered as valid by any operator, regardless of their respective controllerClasses.