Added comment in docs explaining how to use a cell as header in a table

Closes #3437166

core/modules/system/templates/table.html.twig

@@ -30,6 +30,8 @@
  *     - content: The string to display in the table cell.
  *     - active_table_sort: A boolean indicating whether the cell is the active
          table sort.
+ *     - header: Boolean indicating whether the cell should be rendered as a
+ *       header (<th>) or not (<td>).
  * - footer: Table footer rows, in the same format as the rows variable.
  * - empty: The message to display in an extra row if table does not have
  *   any rows.