All of the celebrity tributes and voice-over videos were artificialities, junk, compared to the audience’s acclaim for Serena Williams, the pure roaring waterfalls of applause that thundered down in Arthur Ashe Stadium. It came in full-throated cascades, for the greatest women’s tennis player in history, for the bravura thrust of her game and for the breadth of her dominion now complete. Beneath the shouting and stomping were so many sentiments they could hardly be expressed as the 40-year-old made one last twirl and wave before exiting.