子页面左上角返回文字说明

之前多次写过这个，不过经常忘记，记录一下

let vc = TokenVC()
self.navigationItem.backBarButtonItem = UIBarButtonItem(title: "我", style: .done, target: nil, action: nil) // 重点
vc.hidesBottomBarWhenPushed = true
navigationController?.pushViewController(vc, animated: true)