docs: def params_to_mongo(query_params):
      """Convert HTTP query params to mongodb query syntax.
  
      Converts the parse query params into a mongodb spec.
  
      :param dict query_params: return of func:`rest.process_params`
      """
      if not query_params:
          return {}
      for key, value in query_params.items():
          if isinstance(value, list):
              query_params[key] = {'$in': value}
      return query_params