Add documentation to tbl_ard_summary()

[ddsjoberg]

Make it clear when we do and do NOT need items like.

ard_stack(
  .missing, 
  .attributes,
  .total_n
)

Missing is needed for reporting tbl_ard_summary(missing = c("ifany", "always")).
Attributes are used to improve the default labels by using the column labels.
Total N adds the N to the header internal data frame so it can dynamically be added to the headers.