vorbiscomment: Add DESCRIPTION to ff_vorbiscomment_metadata_conv

It's the official (or recommended) name for comment/description entries.
See https://www.xiph.org/vorbis/doc/v-comment.html

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/vorbiscomment.c

@@ -34,6 +34,7 @@ const AVMetadataConv ff_vorbiscomment_metadata_conv[] = {
     { "ALBUMARTIST", "album_artist"},
     { "TRACKNUMBER", "track"  },
     { "DISCNUMBER",  "disc"   },
+    { "DESCRIPTION", "comment" },
     { 0 }
 };