docs: add example for async addVitePlugin

Author: danielroe

docs/4.api/5.kit/14.builder.md

@@ -207,7 +207,18 @@ See [Vite website](https://vite.dev/guide/api-plugin) for more information about
 
 ### Parameters
 
-**`pluginOrGetter`**: A Vite plugin instance or an array of Vite plugin instances. If a function is provided, it must return a Vite plugin instance or an array of Vite plugin instances.
+**`pluginOrGetter`**: A Vite plugin instance or an array of Vite plugin instances. If a function is provided, it must return a Vite plugin instance or an array of Vite plugin instances. The function can also be async or return a Promise, which is useful for lazy-loading plugins:
+
+```ts twoslash
+import { addVitePlugin, defineNuxtModule } from '@nuxt/kit'
+
+export default defineNuxtModule({
+  setup () {
+    // Lazy load the plugin - only imported when the build actually runs
+    addVitePlugin(() => import('my-vite-plugin').then(r => r.default()))
+  },
+})
+```
 
 **`options`**: Options to pass to the callback function. This object can have the following properties:
 
@@ -266,7 +277,7 @@ See [webpack website](https://webpack.js.org/concepts/plugins/) for more informa
 
 ### Parameters
 
-**`pluginOrGetter`**: A webpack plugin instance or an array of webpack plugin instances. If a function is provided, it must return a webpack plugin instance or an array of webpack plugin instances.
+**`pluginOrGetter`**: A webpack plugin instance or an array of webpack plugin instances. If a function is provided, it must return a webpack plugin instance or an array of webpack plugin instances. The function can also be async or return a Promise, enabling lazy-loading of plugins.
 
 **`options`**: Options to pass to the callback function. This object can have the following properties: