doc: mention reverse proxy and include simple example

PR-URL: #59736
Reviewed-By: Yagiz Nizipli <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Gerhard Stöbich <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>

Author: styfle

doc/api/url.md

@@ -1851,6 +1851,16 @@ 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)`
 
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