code: private void checkDoc() {
          if (foundDoc) {
              if (!checkDocWarningEmitted) {
                  env.warning(null, "javadoc.Multiple_package_comments", name());
                  checkDocWarningEmitted = true;
              }
          } else {
              foundDoc = true;
          }
      }
docs: Invoked when a source of package doc comments is located.
  Emits a diagnostic if this is the second one.