📖 Add warning note about hook invocation in SDK

Signed-off-by: kahirokunn <[email protected]>

Author: kahirokunn

docs/book/src/tasks/experimental-features/runtime-sdk/index.md

@@ -5,6 +5,7 @@ The Runtime SDK feature provides an extensibility mechanism that allows systems,
 <aside class="note warning">
 
 All currently implemented hooks require to also enable the [ClusterClass](../cluster-class/index.md) feature.
+Please note that hooks are only invoked for Clusters created using ClusterClass.
 
 </aside>