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.
@@ -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**:
```
