nit: add details CSI migration (#4695)

Author: louisportay

pages/kubernetes/api-cli/managing-storage.mdx

@@ -46,6 +46,8 @@ To identify your current CSI release version, navigate to the [Cockpit interface
 </Message>
 
 The migration moves volumes from the [Instances API](https://www.scaleway.com/en/developers/api/instance/) to the [Scaleway Block Storage API](https://www.scaleway.com/en/developers/api/block/). No volume detachment is expected, only a control plane restart.
+Newer storage classes have been introduced, with `sbs-default` set as the new default. The existing `scw-bssd` and `scw-bssd-retain` storage classes will remain available for now.
+
 
 ### Using the API with curl