lavc: mention that the parser callback never returns an error

libavcodec/avcodec.h

@@ -3858,6 +3858,8 @@ typedef struct AVCodecParser {
     int codec_ids[5]; /* several codec IDs are permitted */
     int priv_data_size;
     int (*parser_init)(AVCodecParserContext *s);
+    /* This callback never returns an error, a negative value means that
+     * the frame start was in a previous packet. */
     int (*parser_parse)(AVCodecParserContext *s,
                         AVCodecContext *avctx,
                         const uint8_t **poutbuf, int *poutbuf_size,