Update src/content/docs/en/guides/upgrade-to/v6.mdx

Co-authored-by: Sarah Rainsberger <5098874+sarah11918@users.noreply.github.com>

Author: florian-lefebvre

src/content/docs/en/guides/upgrade-to/v6.mdx

@@ -285,7 +285,7 @@ const integration = () => {
 
 In Astro 4.0, the `app.render()` signature that allowed passing `routeData` and `locals` as optional arguments was deprecated in favor of a single optional `renderOptions` argument.
 
-Astro 6.0 removes this signature entirely.
+Astro 6.0 removes this signature entirely. Attempting to pass these separate arguments will now cause an error in your project.
 
 #### What should I do?