message: Add `must_use` attribute to `set_default`
  This makes the user aware of if the guard is dropped immediately and
  thus, the tracing does not work.
diff: @@ -57,6 +57,7 @@ where
  /// [`DefaultGuard`]: ../dispatcher/struct.DefaultGuard.html
  #[cfg(feature = "std")]
  #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
  +#[must_use = "Dropping the guard unregisters the subscriber."]
  pub fn set_default<S>(subscriber: S) -> DefaultGuard
  where
  S: Subscriber + Send + Sync + 'static,
  