Add doc (13013)

Signed-off-by: Nester.zhou <ester.zhou@huawei.com>

en/release-notes/changelogs/OpenHarmony_4.0.2.1/changelog-web.md

+# Web Subsystem ChangeLog
+
+Compared with earlier versions, OpenHarmony 4.0.2.1 has the following API changes in its Web subsystem:
+
+## cl.web.1 Parameter Type Change of postMessageEvent
+
+The **postMessageEvent** API supported only the string type. In OpenHarmony 4.0.2.1 and later versions, it also supports the ArrayBuffer type.
+
+**Change Impacts**
+
+The API change is forward compatible. Applications developed based on earlier versions can still use the API, and the original functions are not affected.
+
+**Key API/Component Changes**
+
+- Involved APIs
+
+  postMessageEvent(message: string): void
+
+- Before change
+
+  ```ts
+  postMessageEvent(message: string): void
+  ```
+
+- After change
+
+  ```ts
+  type WebMessage = ArrayBuffer | string
+  postMessageEvent(message: WebMessage): void
+  ```
+
+**Adaptation Guide**
+
+The API change is forward compatible. Applications developed based on earlier versions can still use the API, and the original functions are not affected.
+
+## cl.web.2 Parameter Type Change of onMessageEvent
+
+The **onMessageEvent** API supported only the string type. In OpenHarmony 4.0.2.1 and later versions, it also supports the ArrayBuffer type.
+
+**Change Impacts**
+
+The API change is forward compatible. Applications developed based on earlier versions can still use the API. With the corresponding logic handling added, the original functions are not affected.
+
+**Key API/Component Changes**
+
+- Involved APIs
+
+  onMessageEvent(callback: (result: string) => void): void
+
+- Before change
+
+  ```ts
+  onMessageEvent(callback: (result: string) => void): void
+  ```
+
+- After change
+
+  ```ts
+  type WebMessage = ArrayBuffer | string
+  onMessageEvent(callback: (result: WebMessage) => void): void
+  ```
+
+**Adaptation Guide**
+
+The API change is forward compatible. Applications developed based on earlier versions can still use the API. With the corresponding logic handling added, the original functions are not affected.