Width of the footnote's top border. Set to 0 for no border, or
NULL to leave the border unchanged.
number_format
Number format for the footnote cell.
...
Other properties, passed to set_cell_properties() for the footnote cell.
Details
add_footnote() is retained for compatibility only and is soft-deprecated.
For new code, use add_table_note(), which stores notes separately from table
cells and does not change the number of rows.