avcodec: fix avcodec_encode_audio() documentation.

the previous documentation indicated how many bytes are read from the input,
not how many samples are read.

libavcodec/avcodec.h

@@ -4200,9 +4200,9 @@ void avsubtitle_free(AVSubtitle *sub);
  * Encode an audio frame from samples into buf.
  *
  * @note The output buffer should be at least FF_MIN_BUFFER_SIZE bytes large.
- * However, for PCM audio the user will know how much space is needed
- * because it depends on the value passed in buf_size as described
- * below. In that case a lower value can be used.
+ * However, for codecs with avctx->frame_size equal to 0 (e.g. PCM) the user
+ * will know how much space is needed because it depends on the value passed
+ * in buf_size as described below. In that case a lower value can be used.
  *
  * @param avctx the codec context
  * @param[out] buf the output buffer
@@ -4210,8 +4210,11 @@ void avsubtitle_free(AVSubtitle *sub);
  * @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 avctx.
- * For PCM audio the number of samples read from samples is equal to
- * buf_size * input_sample_size / output_sample_size.
+ * For codecs which have avctx->frame_size equal to 0 (e.g. PCM) the number of
+ * samples read from samples is equal to:
+ * buf_size * 8 / (avctx->channels * av_get_bits_per_sample(avctx->codec_id))
+ * This also implies that av_get_bits_per_sample() must not return 0 for these
+ * codecs.
  * @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.
  */