Clarify FragmentCollectorRead::read_frame(send_fn) closure documentation

src/fragment.rs

@@ -168,6 +168,10 @@ impl<'f, S> FragmentCollectorRead<S> {
   /// Reads a WebSocket frame, collecting fragmented messages until the final frame is received and returns the completed message.
   ///
   /// Text frames payload is guaranteed to be valid UTF-8.
+  ///
+  /// # Arguments
+  ///
+  /// * `send_fn`: Closure must ensure frames are sent by write side of split WebSocket to correctly implement auto-close and auto-pong.
   pub async fn read_frame<R, E>(
     &mut self,
     send_fn: &mut impl FnMut(Frame<'f>) -> R,