ffplay: extend doxy for audio_decode_frame()

ffplay.c

@@ -2066,7 +2066,13 @@ static int synchronize_audio(VideoState *is, int nb_samples)
     return wanted_nb_samples;
 }
 
-/* decode one audio frame and returns its uncompressed size */
+/**
+ * Decode one audio frame and return its uncompressed size.
+ *
+ * The processed audio frame is decoded, converted if required, and
+ * stored in is->audio_buf, with size in bytes given by the return
+ * value.
+ */
 static int audio_decode_frame(VideoState *is, double *pts_ptr)
 {
     AVPacket *pkt_temp = &is->audio_pkt_temp;