Clarify avcodec_encode_audio() vs. PCM audio.

patch by Stefano Sabatini, stefano.sabatini-lala poste it

Originally committed as revision 13445 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -2750,6 +2750,8 @@ int avcodec_parse_frame(AVCodecContext *avctx, uint8_t **pdata,
  * @param[in] samples the input buffer containing the samples
  * The number of samples read from this buffer is frame_size*channels,
  * both of which are defined in \p avctx.
+ * For PCM audio the number of samples read from \p samples is equal to
+ * \p buf_size * input_sample_size / output_sample_size.
  * @return On error a negative value is returned, on success zero or the number
  * of bytes used to encode the data read from the input buffer.
  */