Improved fetch docs in globals.md

Author: lifeisfoo

doc/api/globals.md

@@ -531,6 +531,39 @@ changes:
 
 A browser-compatible implementation of the [`fetch()`][] function.
 
+```mjs
+const res = await fetch('https://nodejs.org/api/documentation.json');
+if (res.ok) {
+  const data = await res.json();
+  console.log(data);
+}
+```
+
+The implementation is based upon [undici](https://undici.nodejs.org), an HTTP/1.1 client written from scratch for Node.js. You can figure out which version of `undici` is bundled in your Node.js process reading the `process.versions.undici` property.
+
+## Custom dispatcher
+
+You can use a custom dispatcher to dispatch requests passing it in fetch's options object.
+The dispatcher must be compatible with `undici`'s [`Dispatcher` class](https://undici.nodejs.org/#/docs/api/Dispatcher.md).
+
+```js
+fetch(url, { dispatcher: new MyAgent() });
+```
+
+It is possible to change the global dispatcher in Node.js using:
+
+```js
+globalThis[Symbol.for('undici.globalDispatcher.1')] = yourDispatcher;
+```
+
+## Related classes
+
+The following globals are available to use with `fetch`:
+- [`FormData`](https://nodejs.org/api/globals.html#class-formdata)
+- [`Headers`](https://nodejs.org/api/globals.html#class-headers)
+- [`Request`](https://nodejs.org/api/globals.html#request)
+- [`Response`](https://nodejs.org/api/globals.html#response).
+
 ## Class: `File`
 
 <!-- YAML