Change the title of spoiler button by state

The title will give the precise representation of the current state. It
would be helpful for blind people.

app/javascript/mastodon/locales/zh-CN.json

@@ -61,7 +61,8 @@
   "compose_form.publish": "嘟嘟",
   "compose_form.publish_loud": "{publish}！",
   "compose_form.sensitive": "将媒体文件标记为敏感内容",
-  "compose_form.spoiler": "折叠嘟文内容",
+  "compose_form.spoiler.marked": "Text is hidden behind warning",
+  "compose_form.spoiler.unmarked": "Text is not hidden",
   "compose_form.spoiler_placeholder": "折叠部分的警告消息",
   "confirmation_modal.cancel": "取消",
   "confirmations.block.confirm": "屏蔽",