docs: add notes about auto imports from composables/ directory

docs/content/2.guide/3.directory-structure/5.composables.md

@@ -16,14 +16,32 @@ For example:
 
 ```bash
 composables
- | - useFoo.ts
+ | - useFoo.ts // scanned
  | - useBar
- | --- supportingFile.ts
- | --- index.ts
+ | --- supportingFile.ts // not scanned
+ | --- index.ts // scanned
 ```
 
 Only `useFoo.ts` and `useBar/index.ts` would be searched for imports - and if the latter is a default export, it would be registered as `useBar` rather than `index`.
 
+To get auto imports for `useBar/supportingFile.ts`, you have to re-export the composables you need from the `useBar/index.ts` file.
+
+```js [composables/useBar/index.ts]
+export default const useBar () {
+}
+
+// Enables auto import for this export
+export { useBaz } from './supportingFile'
+```
+
+::alert{type=warning}
+Auto import generating doesn't work with `export * from './supportingFile.ts'`, you must use named exports or a default export.
+::
+
+Under the hood, Nuxt auto generates the file `.nuxt/auto-imports.d.ts` to declare the types.
+
+Be aware that you have to run `nuxi prepare`, `nuxi dev` or `nuxi build` in order to let Nuxt generates the types. If you create a composable without having the dev server running, typescript will throw an error `Cannot find name 'useBar'.`
+
 ## Example: (using named export)
 
 ```js [composables/useFoo.ts]