Clarify that plugin may return OK for ControllerUnpublish if node or volume not found

Author: davidz627

spec.md

@@ -1257,6 +1257,9 @@ This RPC is typically called by the CO when the workload using the volume is bei
 
 This operation MUST be idempotent.
 If the volume corresponding to the `volume_id` is not attached to the node corresponding to the `node_id`, the Plugin MUST reply `0 OK`.
+If the volume corresponding to `volume_id` or the node corresponding to
+`node_id` cannot be found by the Plugin and can be safely regarded as
+ControllerUnpublished, the plugin MAY return `0 OK`.
 If this operation failed, or the CO does not know if the operation failed or not, it can choose to call `ControllerUnpublishVolume` again.
 
 ```protobuf