Add comments about the Audio parts of the public Call API being WIP.

BUG=webrtc:4690
R=kjellander@webrtc.org, tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1493933003 .

Cr-Original-Commit-Position: refs/heads/master@{#10882}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a4527c89e77ae6c8c07c935e960bc8d9ddff35b3
diff --git a/audio_receive_stream.h b/audio_receive_stream.h
index 3e5a518..356a3a3 100644
--- a/audio_receive_stream.h
+++ b/audio_receive_stream.h
@@ -24,6 +24,11 @@
 
 class AudioDecoder;
 
+// WORK IN PROGRESS
+// This class is under development and is not yet intended for for use outside
+// of WebRtc/Libjingle. Please use the VoiceEngine API instead.
+// See: https://bugs.chromium.org/p/webrtc/issues/detail?id=4690
+
 class AudioReceiveStream : public ReceiveStream {
  public:
   struct Stats {