Doc: api-ref doc update
Volume multi-attach support was added in cinder and
nova in queens release.
@@ -4,7 +4,9 @@ Volumes (volumes)
=================
A volume is a detachable block storage device similar to a USB hard
-drive. You can attach a volume to one instance at a time.
+drive. You can attach a volume to an instance, and if the volume is
+of an appropriate volume type, a volume can be attached to multiple
+instances.
The ``snapshot_id`` and ``source_volid`` parameters specify the ID
of the snapshot or volume from which this volume originates. If the
