sync: mention lagging in docs for broadcast::send (#5820)

tokio/src/sync/broadcast.rs

@@ -501,7 +501,8 @@ impl<T> Sender<T> {
     ///
     /// On success, the number of subscribed [`Receiver`] handles is returned.
     /// This does not mean that this number of receivers will see the message as
-    /// a receiver may drop before receiving the message.
+    /// a receiver may drop or lag ([see lagging](self#lagging)) before receiving
+    /// the message.
     ///
     /// # Note
     ///