Give some guidance for 404 with catch-all routes

Author: taion

docs/guides/advanced/ServerRendering.md

@@ -27,6 +27,9 @@ serve((req, res) => {
     } else if (redirectLocation) {
       res.redirect(302, redirectLocation.pathname + redirectLocation.search)
     } else if (renderProps) {
+      // You can also check renderProps.components or renderProps.routes for
+      // your "not found" component or route respectively, and send a 404 as
+      // below, if you're using a catch-all route.
       res.status(200).send(renderToString(<RouterContext {...renderProps} />))
     } else {
       res.status(404).send('Not found')