How to control "description_display"?

As this is controlled on a field level I don't think we want to expose any settings. You can control it by adding it programmatically to your fieldset with setting ['#description_display'] to 'before'

See also https://www.drupal.org/node/3143489 →

Ah! Thanks for spotting that.

There's an existing Field Group issue for this: ✨ Add support for #description_display for container types that support it Needs work