code: @Override
      public void addFunctionalInterfaceInfo (Content classInfoTree) {
          if (isFunctionalInterface()) {
              Content dt = HtmlTree.DT(contents.functionalInterface);
              Content dl = HtmlTree.DL(dt);
              Content dd = new HtmlTree(HtmlTag.DD);
              dd.addContent(contents.functionalInterfaceMessage);
              dl.addContent(dd);
              classInfoTree.addContent(dl);
          }
      }
docs: {@inheritDoc}