Mention how to interrupt scrcpy without video

There is no window to close if video playback is disabled.

doc/audio.md

@@ -30,8 +30,9 @@ To disable only the audio playback, see [no playback](video.md#no-playback).
 
 To play audio only, disable the video:
 
-```
+```bash
 scrcpy --no-video
+# interrupt with Ctrl+C
 ```
 
 Without video, the audio latency is typically not criticial, so it might be

doc/video.md

@@ -168,6 +168,7 @@ the computer. This option is useful when [recording](recording.md) or when
 ```bash
 scrcpy --v4l2-sink=/dev/video2 --no-playback
 scrcpy --record=file.mkv --no-playback
+# interrupt with Ctrl+C
 ```
 
 It is also possible to disable video and audio playback separately: