document tag field

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@11647 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/movenc.c

@@ -60,7 +60,7 @@ typedef struct MOVIndex {
     int         hasBframes;
     int         language;
     int         trackID;
-    int         tag;
+    int         tag; ///< stsd fourcc
     AVCodecContext *enc;
 
     int         vosLen;