Add documentation about named lists in `tbl_summary(digits)`

FIRST, we also need to evaluate how we can use the named lists in digits to modify the missing row stats. I think it works if the digits are all named lists (no tidy select, maybe). Minimally it may work with tidyselect, but probably not with the recycling to fill the vector length.

Once we figure out what code will and will not work, then we can update the documentation