MessageBox: Add attribute 'showClose' to doc (#8516)

* 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.

examples/docs/en-US/message-box.md

@@ -380,6 +380,7 @@ The corresponding methods are: `MessageBox`, `MessageBox.alert`, `MessageBox.con
 | type | message type, used for icon display | string | success / info / warning / error | — |
 | customClass | custom class name for MessageBox | string | — | — |
 | callback | MessageBox closing callback if you don't prefer Promise | function(action), where action can be 'confirm' or 'cancel', and `instance` is the MessageBox instance. You can access to that instance's attributes and methods | — | — |
+| showClose | whether to show close icon of MessageBox | boolean | — | true |
 | beforeClose | callback before MessageBox closes, and it will prevent MessageBox from closing | function(action, instance, done), where `action` can be 'confirm' or 'cancel'; `instance` is the MessageBox instance, and you can access to that instance's attributes and methods; `done` is for closing the instance | — | — |
 | lockScroll | whether to lock body scroll when MessageBox prompts | boolean | — | true |
 | showCancelButton | whether to show a cancel button | boolean | — | false (true when called with confirm and prompt) |
@@ -399,4 +400,4 @@ The corresponding methods are: `MessageBox`, `MessageBox.alert`, `MessageBox.con
 | inputValidator | validation function for the input. Should returns a boolean or string. If a string is returned, it will be assigned to inputErrorMessage | function | — | — |
 | inputErrorMessage | error message when validation fails | string | — | Illegal input |
 | center | whether to align the content in center | boolean | — | false |
-| roundButton | whether to use round button | boolean | — | false |
+| roundButton | whether to use round button | boolean | — | false |

examples/docs/zh-CN/message-box.md

@@ -378,6 +378,7 @@ import { MessageBox } from 'element-ui';
 | type | 消息类型，用于显示图标 | string | success / info / warning / error | — |
 | customClass | MessageBox 的自定义类名 | string | — | — |
 | callback | 若不使用 Promise，可以使用此参数指定 MessageBox 关闭后的回调 | function(action, instance)，action 的值为'confirm'或'cancel', instance 为 MessageBox 实例，可以通过它访问实例上的属性和方法 | — | — |
+| showClose | MessageBox 是否显示右上角关闭按钮 | boolean | — | true |
 | beforeClose | MessageBox 关闭前的回调，会暂停实例的关闭 | function(action, instance, done)，action 的值为'confirm'或'cancel'；instance 为 MessageBox 实例，可以通过它访问实例上的属性和方法；done 用于关闭 MessageBox 实例 | — | — |
 | lockScroll | 是否在 MessageBox 出现时将 body 滚动锁定 | boolean | — | true |
 | showCancelButton | 是否显示取消按钮 | boolean | — | false（以 confirm 和 prompt 方式调用时为 true） |