docs: protected void addDisplayType(
          CmsXmlContentDefinition contentDefinition,
          String elementName,
          DisplayType displayType)
      throws CmsXmlException {
  
          if (contentDefinition.getSchemaType(elementName) == null) {
              throw new CmsXmlException(
                  Messages.get().container(Messages.ERR_XMLCONTENT_CONFIG_ELEM_UNKNOWN_1, elementName));
          }
          m_displayTypes.put(elementName, displayType);
      }