Update audio recording documentation

Document how to record audio-only to .opus and .aac files.

PR #3978 <https://github.com/Genymobile/scrcpy/pull/3978>

doc/recording.md

@@ -16,7 +16,9 @@ scrcpy --no-audio --record=file.mp4
 To record only the audio:
 
 ```bash
-scrcpy --no-video --record=file.mp4
+scrcpy --no-video --record=file.opus
+scrcpy --no-video --audio-codec=aac --record-file=file.aac
+# .m4a/.mp4 and .mka/.mkv are also supported for both opus and aac
 ```
 
 To disable mirroring while recording: