This is either not supported at all through .things files or it’s missing from the Things configuration docs. The @ is used to set the location, not the description (note that the location property of the Thing has limited utility.
If you do figure this out, at a minimum please file an issue so it can be added to the docs: How to file an Issue or, if it’s a simple change even you could contribute to the docs. There’s a link at the bottom of the page to a page that will walk you through the steps necessary to create a PR to suggest the change.
Unfortunately I’ve long since given up on file based configs because fighting with the syntax isn’t a problem through managed Things.
An interesting idea just occurred to me. It is possible to create rules in JSON format (same format you see when you query for a rule in the REST API). I wonder if Items and Things can be defined in JSON too.