code: def get_documentation(self, element, namespace=None, schema_str=None):
          
          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 ''
docs: **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