docs: Update doc for extendPageData

Author: kefranabg

packages/docs/docs/plugin/option-api.md

@@ -280,7 +280,7 @@ import { SOURCE_DIR } from '@dynamic/constants'
 
 ## extendPageData
 
-- Type: `Function`
+- Type: `Function|AsyncFunction`
 - Default: `undefined`
 
 A function used to extend or modify the [$page](../guide/global-computed.md#page) object. This function will be invoking once for each page at compile time.
@@ -308,6 +308,16 @@ module.exports = {
 }
 ```
 
+Note that `extendPageData` can also be defined as an asynchronous function.
+
+```js
+module.exports = {
+  async extendPageData ($page) {
+    $page.xxx = await getAsyncData()
+  }
+}
+```
+
 ::: warning Note
 These fields starting with an `_` means you can only access them during build time.
 :::