Update documentation for audio playback capture

PR #5102 <https://github.com/Genymobile/scrcpy/pull/5102>

doc/audio.md

@@ -66,6 +66,30 @@ the computer:
 scrcpy --audio-source=mic --no-video --no-playback --record=file.opus
 ```
 
+### Duplication
+
+An alternative device audio capture method is also available (only for Android
+13 and above):
+
+```
+scrcpy --audio-source=playback
+```
+
+This audio source supports keeping the audio playing on the device while
+mirroring, with `--audio-dup`:
+
+```bash
+scrcpy --audio-source=playback --audio-dup
+# or simply:
+scrcpy --audio-dup  # --audio-source=playback is implied
+```
+
+However, it requires Android 13, and Android apps can opt-out (so they are not
+captured).
+
+
+See [#4380](https://github.com/Genymobile/scrcpy/issues/4380).
+
 
 ## Codec