docs: document handling of local volumes on missing nodes

This is intentionally a separate section because although it applies
to distributed provisioning, the same problem also arises when a CSI
driver handles provisioning of local volumes differently.

README.md

@@ -333,6 +333,21 @@ instead implement a custom policy in a separate controller which sets
 the "selected node" annotation to trigger local provisioning on the
 desired node.
 
+### Deleting local volumes after a node failure or removal
+
+When a node with local volumes gets removed from a cluster before
+deleting those volumes, the PV and PVC objects may still exist. It may
+be possible to remove the PVC normally if the volume was not in use by
+any pod on the node, but normal deletion of the volume and thus
+deletion of the PV is not possible anymore because the CSI driver
+instance on the node is not available or reachable anymore and therefore
+Kubernetes cannot be sure that it is okay to remove the PV.
+
+When an administrator is sure that the node is never going to come
+back, then this situation can be resolved manually:
+- force-delete objects: `kubectl delete pv <pv> --wait=false --grace-period=0 --force`
+- remove all finalizers: `kubectl patch pv <pv> -p '{"metadata":{"finalizers":null}}`
+
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