message: error: fix example of using TracedError with error kind pattern
diff: @@ -17,11 +17,26 @@ struct Erased;
  ///
  /// ```rust,compile_fail
  /// #[derive(Debug, thiserror::Error)]
  +/// enum Kind {
  +/// // ...
  +/// }
  +///
  +/// #[derive(Debug)]
  /// pub struct Error {
  /// source: TracedError<Kind>,
  /// backtrace: Backtrace,
  /// }
  ///
  +/// impl std::error::Error for Error {
  +/// fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
  +/// self.source.source()
  +/// }
  +///
  +/// fn backtrace(&self) -> Option<&Backtrace> {
  +/// Some(&self.backtrace)
  +/// }
  +/// }
  +///
  /// impl fmt::Display for Error {
  /// fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
  /// fmt::Display::fmt(&self.source, fmt)
  