If the active cell is empty: goes to the beginning of the table. Otherwise: first press goes to beginning of the active cell, second press goes to beginning of the current table, third press goes to beginning of document.
如果当前单元格是空白的：跳至表格的起始位置。否则：按第一次时跳至当前单元格的起始位置，按第二次时到当前表格的起始位置，按第三次时到文档的起始位置。