docs: Add FAQ of form onFieldsChange (#23808)

components/form/index.en-US.md

@@ -325,6 +325,16 @@ In most case, we always recommend to use Form `initialValues`. Use Item `initial
 1. Form `initialValues` is the first priority
 2. Field `initialValue` is secondary \*. Not work when multiple Item with same `name` setting the `initialValue`
 
+### Why `onFieldsChange` trigger three times on change when field set `rules`?
+
+Validating is also part of the value updating. It pass follow steps:
+
+1. Trigger value change
+2. Rule validating
+3. Rule validated
+
+In each `onFieldsChange`, you will get `false` > `true` > `false` with `isFieldValidating`.
+
 <style>
   .site-form-item-icon {
     color: rgba(0, 0, 0, 0.25);

components/form/index.zh-CN.md

@@ -326,6 +326,16 @@ validator(rule, value, callback) => {
 1. Form 的 `initialValues` 拥有最高优先级
 2. Field 的 `initialValue` 次之 \*. 多个同 `name` Item 都设置 `initialValue` 时，则 Item 的 `initialValue` 不生效
 
+### 为什么字段设置 `rules` 后更改值 `onFieldsChange` 会触发三次？
+
+字段除了本身的值变化外，校验也是其状态之一。因而在触发字段变化会经历以下几个阶段：
+
+1. Trigger value change
+2. Rule validating
+3. Rule validated
+
+在触发过程中，调用 `isFieldValidating` 会经历 `false` > `true` > `false` 的变化过程。
+
 <style>
   .site-form-item-icon {
     color: rgba(0, 0, 0, 0.25);