docs: def add_description(self, description):
          """Adds a description.
  
          arg:    description (displayText): the new description
          raise:  InvalidArgument - ``description`` is invalid
          raise:  NoAccess - ``Metadata.isReadOnly()`` is ``true``
          raise:  NullArgument - ``description`` is ``null``
          *compliance: mandatory -- This method must be implemented.*
  
          """
          if self.get_descriptions_metadata().is_read_only():
              raise NoAccess()
          if not isinstance(description, DisplayText):
              raise InvalidArgument('description must be instance of DisplayText')
          self.add_or_replace_value('descriptions', description)