Update custom server docs for async methods (vercel#30521)

as handle() returned from app.getRequestHandler() is a Promise, it has to be handled correctly. Simply returning it, works fine.


## Bug
- [x] Related issues linked using fixes #number
- [x] Integration tests added
- [x] Errors have helpful link attached, see contributing.md

## Feature
- [x] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [x] Related issues linked using fixes #number
- [x] Integration tests added
- [x] Documentation added
- [x] Telemetry added. In case of a feature if it's used or not.
- [x] Errors have helpful link attached, see contributing.md

## Documentation / Examples

- [x] Make sure the linting passes by running `yarn lint`


Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

docs/advanced-features/custom-server.md

@@ -36,18 +36,24 @@ const app = next({ dev, hostname, port })
 const handle = app.getRequestHandler()
 
 app.prepare().then(() => {
-  createServer((req, res) => {
-    // Be sure to pass `true` as the second argument to `url.parse`.
-    // This tells it to parse the query portion of the URL.
-    const parsedUrl = parse(req.url, true)
-    const { pathname, query } = parsedUrl
-
-    if (pathname === '/a') {
-      app.render(req, res, '/a', query)
-    } else if (pathname === '/b') {
-      app.render(req, res, '/b', query)
-    } else {
-      handle(req, res, parsedUrl)
+  createServer(async (req, res) => {
+    try {
+      // Be sure to pass `true` as the second argument to `url.parse`.
+      // This tells it to parse the query portion of the URL.
+      const parsedUrl = parse(req.url, true)
+      const { pathname, query } = parsedUrl
+
+      if (pathname === '/a') {
+        await app.render(req, res, '/a', query)
+      } else if (pathname === '/b') {
+        await app.render(req, res, '/b', query)
+      } else {
+        await handle(req, res, parsedUrl)
+      }
+    } catch (err) {
+      console.error('Error occurred handling', req.url, err)
+      res.statusCode = 500
+      res.end('internal server error')
     }
   }).listen(port, (err) => {
     if (err) throw err