doc: added more info about nested fields to migration guide (#21758)

Added more examples to form nested fields.

components/form/v3.en-US.md

@@ -265,6 +265,49 @@ form.getFieldsError();
 */
 ```
 
+Nested field definition has changed from:
+
+```jsx
+// antd v3
+<Form.Item label="Firstname">
+  {getFieldDecorator("user.0.firstname", {})(<Input />)}
+</Form.Item>
+```
+
+To 
+
+```jsx
+// antd v4
+<Form.Item
+  name={['user', '0', 'firstname']}
+  label="Firstname"
+>
+  <Input />
+</Form.Item>
+```
+
+Similarly using `setFieldsValue` has changed from:
+
+```jsx
+// antd v3
+this.formRef.current.setFieldsValue({
+  'user.0.firstname': 'John'
+});
+```
+
+To
+
+```jsx
+// antd v4
+this.formRef.current.setFieldsValue({
+  user: 
+    [{
+      firstname: 'John'
+    }]
+});
+```
+
+
 ### Remove callback in validateFields
 
 `validateFields` will return a Promise, so you can handle the error with `async/await` or `then/catch`. It is no longer necessary to determine if `errors` is empty:

components/form/v3.zh-CN.md

@@ -267,6 +267,49 @@ form.getFieldsError();
 */
 ```
 
+
+嵌套字段定义由：
+
+```jsx
+// antd v3
+<Form.Item label="Firstname">
+  {getFieldDecorator("user.0.firstname", {})(<Input />)}
+</Form.Item>
+```
+
+改至：
+
+```jsx
+// antd v4
+<Form.Item
+  name={['user', '0', 'firstname']}
+  label="Firstname"
+>
+  <Input />
+</Form.Item>
+```
+
+相似的，`setFieldsValue` 由：
+
+```jsx
+// antd v3
+this.formRef.current.setFieldsValue({
+  'user.0.firstname': 'John'
+});
+```
+
+改至：
+
+```jsx
+// antd v4
+this.formRef.current.setFieldsValue({
+  user: 
+    [{
+      firstname: 'John'
+    }]
+});
+```
+
 ## validateFields 不再支持 callback
 
 `validateFields` 会返回 Promise 对象，因而你可以通过 `async/await` 或者 `then/catch` 来执行对应的错误处理。不再需要判断 `errors` 是否为空：