Document change in builtin variables from 12235

Author: fabriziopandini

docs/book/src/developer/providers/migrations/v1.10-to-v1.11.md

@@ -134,6 +134,8 @@ proposal because most of the changes described below are a consequence of the wo
 ### ClusterClass
 
 - See changes that apply to [all CRDs](#all-crds)
+- The `builtin.cluster.classRef.Name` and `builtin.cluster.classRef.Namespace` variables has been added
+  - The `builtin.cluster.class` and `builtin.cluster.classNamespace` are deprecated and will be removed with the next apiVersion. 
 - The `builtin.cluster.network.ipFamily` variable has been removed and it cannot be used anymore in patches
 
 ### KubeadmConfig