Add docstring.

TodoList.py

@@ -234,6 +234,7 @@ class TodoList(object):
         order).
         """
         def clean_by_tag(tag_name):
+            """ Generic function to handle 'p' and 'id' tags. """
             for todo in [todo for todo in self._todos if todo.has_tag(tag_name)]:
                 value = todo.tag_value(tag_name)
                 if not self._depgraph.has_edge_id(value):