Update lib.rs

Add a comment to highlight the code flow. I was a bit stuck on this example and thought it had an error. However, when I found the `return` keyword, I understood the true flow. I would like to add this comment to make the flow of reading smoother.

examples/02_02_future_trait/src/lib.rs

@@ -125,6 +125,7 @@ where
                 // the second!
                 Poll::Ready(()) => self.first.take(),
                 // We couldn't yet complete the first future.
+                // Notice that we disrupt the flow of the `pool` function with the `return` statement.
                 Poll::Pending => return Poll::Pending,
             };
         }