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-Commit-Position: refs/heads/master@{#10882}

webrtc/audio_receive_stream.h

@@ -24,6 +24,11 @@ namespace webrtc {
 
 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 {

webrtc/audio_send_stream.h

@@ -23,6 +23,11 @@
 
 namespace webrtc {
 
+// 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 AudioSendStream : public SendStream {
  public:
   struct Stats {

webrtc/audio_state.h

@@ -18,6 +18,11 @@ namespace webrtc {
 class AudioDeviceModule;
 class VoiceEngine;
 
+// 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
+
 // AudioState holds the state which must be shared between multiple instances of
 // webrtc::Call for audio processing purposes.
 class AudioState : public rtc::RefCountInterface {