Document padding for AVProbeData.

Patch by roundup user shinya, tesutosan hotmail


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@19802 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/avformat.h

@@ -145,8 +145,8 @@ struct AVCodecTag;
 /** This structure contains the data a format has to probe a file. */
 typedef struct AVProbeData {
     const char *filename;
-    unsigned char *buf;
-    int buf_size;
+    unsigned char *buf; /**< Buffer must have AVPROBE_PADDING_SIZE of extra allocated bytes filled with zero. */
+    int buf_size;       /**< Size of buf except extra allocated bytes */
 } AVProbeData;
 
 #define AVPROBE_SCORE_MAX 100               ///< maximum score, half of that is used for file-extension-based detection