Patch by Anton Khirnov <wyskas gmail com>.
Originally committed as revision 22028 to svn://svn.ffmpeg.org/ffmpeg/trunk
/*
* Public Metadata API.
* The metadata API allows libavformat to export metadata tags to a client
- * application using a sequence of key/value pairs.
+ * application using a sequence of key/value pairs. Like all strings in FFmpeg,
+ * metadata must be stored as UTF-8 encoded Unicode. Note that metadata
+ * exported by demuxers isn't checked to be valid UTF-8 in most cases.
* Important concepts to keep in mind:
* 1. Keys are unique; there can never be 2 tags with the same key. This is
* also meant semantically, i.e., a demuxer should not knowingly produce