* MessageBox: Add attribute 'showClose' to doc

Add an attribute 'showClose' to explain whether to show 'close' button on the right top.

MessageBox: 文档中添加一个属性：showClose。 用来解释是否显示 MessageBox 右上角的关闭按钮。改属性，已经在源码中声明。

* MessageBox: Add attribute 'showClose' to doc

Add an attribute 'showClose' to explain whether to show 'close' button on the right top.