docs: add info about merging strategy in app config (#21462)

docs/2.guide/2.directory-structure/3.app-config.md

@@ -91,3 +91,32 @@ declare module 'nuxt/schema' {
 // It is always important to ensure you import/export something when augmenting a type
 export {}
 ```
+
+### App Config Merging Strategy in Layers
+
+Nuxt uses a custom merging strategy for the `AppConfig` within the layers of your application.
+This strategy is implemented using a [Function Merger](https://github.com/unjs/defu#function-merger), which allows defining a custom merging strategy for every key in `app.config` that has an array as value.
+
+::alert{type=warning}
+The Function Merger should only be used in the base `app.config` of your application.
+::
+
+Here's an example of how you can use:
+
+::code-group
+
+```ts [layer/app.config.ts]
+export default defineAppConfig({
+  // Default array value
+  array: ['hello'],
+})
+```
+
+```ts [app.config.ts]
+export default defineAppConfig({
+  // Overwrite default array value by using a merger function
+  array: () => ['bonjour'],
+})
+```
+
+::