docs(Form): How to use ref with Form HOC (#19937)

* 为Form新增如何在函数组件中使用的例子

* 在Form FAQ中增加在函数组件中怎么正确拿到Form实例的说明

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components/form/index.en-US.md

@@ -275,3 +275,38 @@ validator(rule, value, callback) => {
   }
 }
 ```
+
+### Get form instance from function component
+
+You can combine `forwardRef` with `useImperativeHandle` to get form instance:
+
+```tsx
+import React, { forwardRef, useImperativeHandle } from 'react';
+import Form, { FormComponentProps } from 'antd/lib/form/Form';
+
+const FCForm = forwardRef<FormComponentProps, FCFormProps>(({ form, onSubmit }, ref) => {
+  useImperativeHandle(ref, () => ({
+    form,
+  }));
+  `...the rest of your form`;
+});
+const EnhancedFCForm = Form.create<FCFormProps>()(FCForm);
+```
+
+You can use your form component like this：
+
+```tsx
+const TestForm = () => {
+  const formRef = createRef<Ref>();
+  return (
+    <EnhancedFCForm
+      onSubmit={() => console.log(formRef.current!.form.getFieldValue('name'))}
+      wrappedComponentRef={formRef}
+    />
+  );
+};
+```
+
+Online demo:
+
+[![Edit wrappedComponentRef-in-function-component](https://codesandbox.io/static/img/play-codesandbox.svg)](https://codesandbox.io/s/wrappedcomponentref-in-function-component-fj43c?fontsize=14&hidenavigation=1&theme=dark)

components/form/index.zh-CN.md

@@ -277,3 +277,38 @@ validator(rule, value, callback) => {
   }
 }
 ```
+
+### 如何在函数组件中拿到 form 实例？
+
+你需要通过 `forwardRef` 和 `useImperativeHandle` 的组合使用来实现在函数组件中正确拿到 form 实例：
+
+```tsx
+import React, { forwardRef, useImperativeHandle } from 'react';
+import Form, { FormComponentProps } from 'antd/lib/form/Form';
+
+const FCForm = forwardRef<FormComponentProps, FCFormProps>(({ form, onSubmit }, ref) => {
+  useImperativeHandle(ref, () => ({
+    form,
+  }));
+  `...the rest of your form`;
+});
+const EnhancedFCForm = Form.create<FCFormProps>()(FCForm);
+```
+
+使用表单组件可以写成这样：
+
+```tsx
+const TestForm = () => {
+  const formRef = createRef<Ref>();
+  return (
+    <EnhancedFCForm
+      onSubmit={() => console.log(formRef.current!.form.getFieldValue('name'))}
+      wrappedComponentRef={formRef}
+    />
+  );
+};
+```
+
+在线示例：
+
+[![Edit wrappedComponentRef-in-function-component](https://codesandbox.io/static/img/play-codesandbox.svg)](https://codesandbox.io/s/wrappedcomponentref-in-function-component-fj43c?fontsize=14&hidenavigation=1&theme=dark)