doc: clarify difference between Node.js fetch and browser fetch

doc/api/globals.md

@@ -557,6 +557,25 @@ The implementation is based upon [undici](https://undici.nodejs.org), an HTTP/1.
 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.
 
+### Differences from the Web Standard `fetch()`
+
+Node.js implements `fetch()` using the **Undici** HTTP client
+rather than a browser networking stack. Although the API shape
+follows the WHATWG Fetch specification, some browser-specific
+behaviors do not apply in Node.js environments.
+
+Notable differences:
+
+* Browser features such as service workers, cache modes,
+  and referrer policies are not supported.
+* CORS restrictions do not apply in Node.js.
+* Networking behavior (TLS, proxies, connection pooling)
+  follows Node.js rules via Undici.
+* Some options accepted by browser `fetch()` may be ignored
+  or behave differently.
+* `Request` and `Response` objects are designed for 
+  server-side usage.
+
 ### Custom dispatcher
 
 You can use a custom dispatcher to dispatch requests passing it in fetch's options object.