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 the volume can be safely regarded as
+ControllerUnpublished from the node, 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
@@ -1292,8 +1295,8 @@ The CO MUST implement the specified error recovery behavior when it encounters t
 
 | Condition | gRPC Code | Description | Recovery Behavior |
 |-----------|-----------|-------------|-------------------|
-| Volume does not exist | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
-| Node does not exist | 5 NOT_FOUND | Indicates that a node corresponding to the specified `node_id` does not exist. | Caller MUST verify that the `node_id` is correct and that the node is available and has not been terminated or deleted before retrying with exponential backoff. |
+| Volume does not exist and volume not assumed ControllerUnpublished from node | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist and volume not assumed ControllerUnpublished from node. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
+| Node does not exist and volume not assumed ControllerUnpublished from node  | 5 NOT_FOUND | Indicates that a node corresponding to the specified `node_id` does not exist and volume not assumed ControllerUnpublished from node. | Caller MUST verify that the `node_id` is correct and that the node is available and has not been terminated or deleted before retrying with exponential backoff. |
 
 
 #### `ValidateVolumeCapabilities`