extend description of avcodec_decode_audio3:

- clarify when 0 is returned
- explain that decode_audio3 has to be called multiple times when there
  are multiple frames in a single packet

Originally committed as revision 20215 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -3228,9 +3228,11 @@ attribute_deprecated int avcodec_decode_audio2(AVCodecContext *avctx, int16_t *s
 /**
  * Decodes the audio frame of size avpkt->size from avpkt->data into samples.
  * Some decoders may support multiple frames in a single AVPacket, such
- * decoders would then just decode the first frame.
+ * decoders would then just decode the first frame. In this case,
+ * avcodec_decode_audio3 has to be called again with an AVPacket that contains
+ * the remaining data in order to decode the second frame etc.
  * If no frame
- * could be decompressed, frame_size_ptr is zero. Otherwise, it is the
+ * could be outputted, frame_size_ptr is zero. Otherwise, it is the
  * decompressed frame size in bytes.
  *
  * @warning You must set frame_size_ptr to the allocated size of the
@@ -3260,7 +3262,7 @@ attribute_deprecated int avcodec_decode_audio2(AVCodecContext *avctx, int16_t *s
  *            data and size, some decoders might in addition need other fields.
  *            All decoders are designed to use the least fields possible though.
  * @return On error a negative value is returned, otherwise the number of bytes
- * used or zero if no frame could be decompressed.
+ * used or zero if no frame data was decompressed (used) from the input AVPacket.
  */
 int avcodec_decode_audio3(AVCodecContext *avctx, int16_t *samples,
                          int *frame_size_ptr,