[api-ref] Add backing-up to snapshot statuses
Now snapshot contains a new status called backing-up
Cloese-bug:
@@ -18,6 +18,8 @@ possible:
+----------------+-------------------------------------+
| available | The snapshot is ready to use. |
+----------------+-------------------------------------+
+| backing-up | The snapshot is being backed up. |
++----------------+-------------------------------------+
| deleting | The snapshot is being deleted. |
+----------------+-------------------------------------+
| error | A snapshot creation error occurred. |
