Update examples/webrtc_dummy.rs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

examples/webrtc_dummy.rs

@@ -4,6 +4,8 @@ use bytes::BytesMut;
 
 use hbb_common::ResultType;
 
+/// Dummy implementation of WebRTCStream used when the `webrtc` feature is disabled.
+/// This struct allows the code to compile and run without actual WebRTC functionality.
 pub struct WebRTCStream {
     // mock struct
 }