restore migration guide for MessageBox

Author: MarcusNotheis

docs/MigrationGuide.mdx

@@ -263,6 +263,35 @@ function MyComponent() {
 }
 ```
 
+### MessageBox
+
+- `onClose` is now a plain callback function and no `CustomEvent` anymore. It receives two parameters, `action` and `escPressed`.
+
+```jsx
+// v1
+// onClose?: (event: CustomEvent<{ action: MessageBoxAction }>) => void;
+
+<MessageBox
+  onClose={(event) => {
+    console.log(event.detail.action);
+  }}
+>
+  {children}
+</MessageBox>
+
+// v2
+// onClose?: (action: MessageBoxActionType | undefined, escPressed?: true) => void;
+
+<MessageBox
+  onClose={(action, escPressed) => {
+    console.log(action, escPressed);
+  }}
+>
+  {children}
+</MessageBox>
+
+```
+
 ### ObjectPage
 
 The newly introduced `DynamicPage` web component comes with its own `DynamicPageHeader` and `DynamicPageTitle` components, which are unfortunately incompatible with our `ObjectPage` implementation.