Updated volume-driver plugin api docs

There was a mistake in the plugin_volume.md file regarding the /VolumeDriver.Remove call.

Added fix and a better explanation.

Signed-off-by: Bardia Keyoumarsi <b.keyoumarsi@gmail.com>

docs/extend/plugins_volume.md

@@ -78,7 +78,7 @@ Respond with a string error if an error occurred.
 }
 ```
 
-Create a volume, given a user specified volume name.
+Delete the specified volume from disk. This request is issued when a user invokes `docker rm -v` to remove volumes associated with a container.
 
 **Response**:
 ```