Document padding for AVProbeData.

Patch by roundup user shinya, tesutosan hotmail

Originally committed as revision 19802 to svn://svn.ffmpeg.org/ffmpeg/trunk

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