multiple mount/unmount req with same volume name

Updating docs to handle mount/unmount requests with same volume name. Issue #17585

Signed-off-by: Moorthy RS <rsmoorthy@gmail.com>

docs/extend/plugins_volume.md

@@ -101,7 +101,9 @@ Respond with a string error if an error occurred.
 ```
 
 Docker requires the plugin to provide a volume, given a user specified volume
-name. This is called once per container start.
+name. This is called once per container start. If the same volume_name is requested
+more than once, the plugin may need to keep track of each new mount request and provision
+at the first mount request and deprovision at the last corresponding unmount request.
 
 **Response**:
 ```