doc: improved fetch docs

doc/api/globals.md

@@ -531,6 +531,46 @@ 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 installing `undici` and using
+the `setGlobalDispatcher()` method. Calling this method will affect both `undici` and
+Node.js.
+
+```mjs
+import { setGlobalDispatcher } from 'undici';
+setGlobalDispatcher(new MyAgent());
+```
+
+## 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`
 
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