Be more explicit about volume removal

Signed-off-by: Peter Salvatore <peter@psftw.com>

docs/sources/userguide/dockervolumes.md

@@ -119,7 +119,8 @@ You can also extend the chain by mounting the volume that came from the
 
 If you remove containers that mount volumes, including the initial `dbdata`
 container, or the subsequent containers `db1` and `db2`, the volumes will not
-be deleted until there are no containers still referencing those volumes. This
+be deleted.  To delete the volume from disk, you must explicitly call
+`docker rm -v` against the last container with a reference to the volume. This
 allows you to upgrade, or effectively migrate data volumes between containers.
 
 ## Backup, restore, or migrate data volumes