Add note to volume docs that data is not copied to mounted volumes

Signed-off-by: Candid Dauth <cdauth@cdauth.eu>

docs/userguide/dockervolumes.md

@@ -29,7 +29,8 @@ containers that bypasses the [*Union File System*](../reference/glossary.md#unio
 
 - Volumes are initialized when a container is created. If the container's
   base image contains data at the specified mount point, that existing data is
-  copied into the new volume upon volume initialization.
+  copied into the new volume upon volume initialization. (Note that this does
+  not apply when [mounting a host directory](#mount-a-host-directory-as-a-data-volume).)
 - Data volumes can be shared and reused among containers.
 - Changes to a data volume are made directly.
 - Changes to a data volume will not be included when you update an image.