code: def add_media_description(self, media_description):
          
          if self.get_media_descriptions_metadata().is_read_only():
              raise NoAccess()
          self.add_or_replace_value('mediaDescriptions', media_description)
docs: Adds a media_description.
  
          arg:    media_description (displayText): the new media_description
          raise:  InvalidArgument - ``media_description`` is invalid
          raise:  NoAccess - ``Metadata.isReadOnly()`` is ``true``
          raise:  NullArgument - ``media_description`` is ``null``
          *compliance: mandatory -- This method must be implemented.*