lavf: update documentation of AVOutputFormat.flags

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavformat/avformat.h

@@ -297,7 +297,9 @@ typedef struct AVOutputFormat {
     int (*write_packet)(struct AVFormatContext *, AVPacket *pkt);
     int (*write_trailer)(struct AVFormatContext *);
     /**
-     * can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER, AVFMT_GLOBALHEADER
+     * can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER, AVFMT_RAWPICTURE,
+     * AVFMT_GLOBALHEADER, AVFMT_NOTIMESTAMPS, AVFMT_VARIABLE_FPS,
+     * AVFMT_NODIMENSIONS, AVFMT_NOSTREAMS
      */
     int flags;
     /**