add some usage doc

docs/usage/usage.md

@@ -287,6 +287,10 @@ Every GCP shoot cluster will be deployed with the GCP PD CSI driver.
 It is compatible with the legacy in-tree volume provisioner that was deprecated by the Kubernetes community and will be removed in future versions of Kubernetes.
 End-users might want to update their custom `StorageClass`es to the new `pd.csi.storage.gke.io` provisioner.
 
+## Support for VolumeAttributeClasses (Beta in k8s 1.31)
+
+To have the CSI-Driver configured to support the necessary features for [VolumeAttributeClasses](https://kubernetes.io/docs/concepts/storage/volume-attributes-classes/) on GCP for shoots with a k8s-version greater than 1.31, use the `gcp.provider.extensions.gardener.cloud/enable-modify-volume` annotation on the shoot. Keep in mind that as of 1.31 you _must_ also configure the `kube-apiserver` to enable the `storage.k8s.io/v1beta1` API group.
+
 ## Kubernetes Versions per Worker Pool
 
 This extension supports `gardener/gardener`'s `WorkerPoolKubernetesVersion` feature gate, i.e., having [worker pools with overridden Kubernetes versions](https://github.com/gardener/gardener/blob/8a9c88866ec5fce59b5acf57d4227eeeb73669d7/example/90-shoot.yaml#L69-L70) since `gardener-extension-provider-gcp@v1.21`.