feat: add migration guide > watch

Author: naokie

src/guide/migration/watch.md

@@ -0,0 +1,30 @@
+---
+title: Watch on Arrays
+badges:
+  - breaking
+---
+
+# {{ $frontmatter.title }} <MigrationBadges :badges="$frontmatter.badges" />
+
+## Overview
+
+- **BREAKING**: When watching an array, the callback will only trigger when the array is replaced. If you need to trigger on mutation, the `deep` option must be specified.
+
+## 3.x Syntax
+
+When using [the `watch` option](/api/options-data.html#watch) to watch an array, the callback will only trigger when the array is replaced. In other words, the watch callback will no longer be triggered on array mutation. To trigger on mutation, the `deep` option must be specified.
+
+```js
+watch: {
+  bookList: {
+    handler(val, oldVal) {
+      console.log('book list changed')
+    },
+    deep: true
+  },
+}
+```
+
+## Migration Strategy
+
+If you rely on watching array mutations, add the `deep` property to ensure that your callback is triggered correctly.