Mark TestADM test API

Bug: b/272350185, webrtc:15081
Change-Id: I461162ed4e4afd111b2c803b2d11161f3e5b93e1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/300863
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39843}
diff --git a/modules/audio_device/include/test_audio_device.h b/modules/audio_device/include/test_audio_device.h
index 8413479..e52e93c 100644
--- a/modules/audio_device/include/test_audio_device.h
+++ b/modules/audio_device/include/test_audio_device.h
@@ -26,6 +26,9 @@
 
 namespace webrtc {
 
+// This is test API and is in development, so it can be changed/removed without
+// notice.
+
 // TestAudioDeviceModule implements an AudioDevice module that can act both as a
 // capturer and a renderer. It will use 10ms audio frames.
 class TestAudioDeviceModule : public AudioDeviceModule {