id3v2: add @file doxy and link to format documentation

libavformat/id3v2.c

 /*
- * ID3v2 header parser
  * Copyright (c) 2003 Fabrice Bellard
  *
  * This file is part of FFmpeg.
@@ -19,6 +18,14 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+/**
+ * @file
+ * ID3v2 header parser
+ *
+ * Specifications available at:
+ * http://id3.org/Developer_Information
+ */
+
 #include "id3v2.h"
 #include "id3v1.h"
 #include "libavutil/avstring.h"