Add custom labels for column Summary items

I’d like to be able to add a text label of “Total” or “Subtotal” (or, ideally, a custom label that I define) to the left of a column summary cell. This seems like a useful feature when delivering a doc to someone else.

In a related vein, I’d also like the ability to add a “tooltip” when hovering over any cell to allow a description of what the cell is showing.