docs: improve enhanceApp usecases (#5315)

docs/en/guide/custom-theme.md

@@ -62,11 +62,14 @@ import Layout from './Layout.vue'
 export default {
   Layout,
   enhanceApp({ app, router, siteData }) {
-    // ...
+    // app.component(...)
+    // app.use(...)
   }
 }
 ```
 
+The `enhanceApp` hook allows you to access the [Vue app instance](https://vuejs.org/api/application.html) and other runtime data, which can be used to [register global components](./extending-default-theme.md#registering-global-components), integrate with Vue libraries, etc.
+
 The `router` value is the same VitePress router instance returned by [`useRouter()`](../reference/runtime-api#userouter). To listen for route changes, assign handlers on the router:
 
 ```ts [.vitepress/theme/index.ts]