recommend against using `unload` event

Currently this article recommends using the unload event to close popups and send stats. unload cannot be relied upon and best practice is to avoid it entirely, using pagehide or visibilitychange.

E.g. popups should be closed in pagehide, not unload because a page that canenter BFCache will not fire unload but will fire pagehide

Stats should not be sent from unload because if the page goes into BFCache and the user never returns, the unload event will never fire.

打开 2-ui/5-loading/01-onload-ondomcontentloaded/article.md，并查看 issue 中链接的 window.onunload 部分。更新建议，使弹窗清理和统计信息指南不再依赖 unload，并确保完成的文章体现 pagehide 或 visibilitychange，以及此处所述的 BFCache 限制。