devices: add missing docstring for _get_device_classes

qubesadmin/devices.py

@@ -309,6 +309,9 @@ class DeviceManager(dict):
         return self._get_device_classes()
 
     def _get_device_classes(self):
+        """Function used to call Qubesd in order to obtain
+        the device classes list
+        """
         device_classes = \
             self._vm.app.qubesd_call('dom0', 'admin.deviceclass.List').decode()
         device_classes = sorted(device_classes.splitlines())