Adding a description/comment to snapshots

Hi!

I've been using btrbk to snapshot and create backups, and it's a fantastic tool. Thank you. :raised_hands:

I've been using btrbk to snapshot pre/post installing packages with Pacman, and I would love to add a description to the snapshot.

I've been reading through the source code and manpages and wikis, and I don't believe it's possible to add a comment/description to a btrfs subvolume, though?

I know [snapper](https://wiki.archlinux.org/title/snapper) has `--description foo`, but I'm not sure where that is saved, and I'm assuming it's some log or something.