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>