def get_documentation(self, element, namespace=None, schema_str=None):
        """**Helper method:** should return an schema specific documentation
        given an element parsing or getting the `Clark's Notation`_
        `{url:schema}Element` from the message error on validate method.

        :param str element: Element string following the Clark's Notation
        :param dict namespace: Element string following the Clark's Notation

        :returns: The documentation text if exists
        :rtype: unicode

        .. _`Clark's Notation`: http://effbot.org/zone/element-namespaces.htm
        """
        if namespace is None:
            namespace = {'xs': 'http://www.w3.org/2001/XMLSchema'}
        schema_root = etree.parse(StringIO(self.schema))
        document = schema_root.xpath(self.get_element_from_clark(element),
                                     namespaces=namespace)
        return document and document[0].text or ''
**Helper method:** should return an schema specific documentation
        given an element parsing or getting the `Clark's Notation`_
        `{url:schema}Element` from the message error on validate method.

        :param str element: Element string following the Clark's Notation
        :param dict namespace: Element string following the Clark's Notation

        :returns: The documentation text if exists
        :rtype: unicode

        .. _`Clark's Notation`: http://effbot.org/zone/element-namespaces.htm