doc: mention reverse proxy and include simple example

PR-URL: #59736
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: styfle

doc/api/url.md

@@ -1857,6 +1857,16 @@ function getURL(req) {
 }
 ```
 
+The example above assumes well-formed headers are forwarded from a reverse
+proxy to your Node.js server. If you are not using a reverse proxy, you should
+use the example below:
+
+```js
+function getURL(req) {
+  return new URL(req.url || '/', 'https://example.com');
+}
+```
+
 ### `url.resolve(from, to)`
 
 <!-- YAML