Alright, RESOLVED.
I wasn’t aware of the “Layout” type fields that could be added to a field group, and which address what I’m asking for by letting me mix for example the “Message” field type into my forms.
By using those fields, I’m able to add a “description” at the top of my field group to explain things to users.
Hopefully this embarrassing post will help someone else in the future who’s googling for a way to add a “description” to a group.