lavc/avcodec: Improve av_parser_parse() documentation, mention padding.

Fixes ticket #5809.

libavcodec/avcodec.h

@@ -5146,7 +5146,10 @@ AVCodecParserContext *av_parser_init(int codec_id);
  * @param poutbuf       set to pointer to parsed buffer or NULL if not yet finished.
  * @param poutbuf_size  set to size of parsed buffer or zero if not yet finished.
  * @param buf           input buffer.
- * @param buf_size      input length, to signal EOF, this should be 0 (so that the last frame can be output).
+ * @param buf_size      buffer size in bytes without the padding. I.e. the full buffer
+                        size is assumed to be buf_size + AV_INPUT_BUFFER_PADDING_SIZE.
+                        To signal EOF, this should be 0 (so that the last frame
+                        can be output).
  * @param pts           input presentation timestamp.
  * @param dts           input decoding timestamp.
  * @param pos           input byte position in stream.