docs(how-to/file-route-conventions): clarify 404 response code in catch-all section (#13771)

Author: davidbielik

docs/how-to/file-route-conventions.md

@@ -301,6 +301,15 @@ To create a route that will match any requests that don't match other defined ro
 | `/about`                       | `app/routes/about.tsx`  |
 | `/any-invalid-path-will-match` | `app/routes/$.tsx`      |
 
+To have this route serve as a 404 page, be sure to modify the response code with a [`loader`](https://reactrouter.com/start/framework/data-loading#server-data-loading) function:
+
+```tsx
+import type { Route } from './+types/$';
+export const loader = async ({ request }: Route.LoaderArgs) => {
+  return data({}, 404);
+};
+```
+
 ## Escaping Special Characters
 
 If you want one of the special characters used for these route conventions to actually be a part of the URL, you can escape the conventions with `[]` characters. This can be especially helpful for [resource routes][resource_routes] that include an extension in the URL.