docs: def format_arrow(value, format_string):
      """Format an arrow datetime object.
  
      :param value: The arrow datetime object.
      :param format_string: The date format string
      :returns: Returns a string representation of the given arrow datetime
          object, formatted according to the given format string.
  
      .. note::
          Do not use this filter to format date/times presented to an end
          user. Instead use ``datetimeformat`` or ``dateformat`` from
          Invenio-I18N.
      """
      assert isinstance(value, arrow.Arrow)
      return value.format(format_string)