The REST API doesn't appear to have an attribute to update the description, and since the Tableau Server Client is built on top of the REST API, I'm nos surprised you don't see it there either. Unless I'm missing something obvious, this might be a good feature for you to submit an idea for. That'd be a good way to get it on Tableau's development team radar.
Yes, this is on our radar, and the team is actively working on it. You can follow the update by subscribing to this Issue on Github: Add Data Source Description · Issue #135 · tableau/server-client-python · GitHub