docs(fetch): add differences between Undici fetch and standard Fetch API

docs/docs/api/Fetch.md

@@ -50,3 +50,21 @@ const busboy = new Busboy({
 
 Readable.fromWeb(response.body).pipe(busboy)
 ```
+
+## Differences Between Undici's Fetch and the Standard Fetch API
+
+The `fetch` implementation in Undici is inspired by the Fetch API standard but has some key differences:
+
+1. **`new Response(asyncIterable)`**:
+   - Undici extends the standard `Response` constructor to accept an `asyncIterable` as its body. This allows streams and other async sources to be directly used. This feature is not part of the Fetch API in browsers.
+
+2. **Cookies Handling**:
+   - Unlike browsers, Undici does not automatically manage cookies. In browsers, cookies are managed through the `Cookie` header and automatically sent with requests. In Undici, you need to manually handle cookies.
+
+3. **No Forbidden Headers**:
+   - In browsers, some headers (e.g., `User-Agent`, `Referer`) are restricted from being modified for security reasons. In Undici, these headers can be freely set.
+
+4. **Environment-Specific Behavior**:
+   - Undici operates outside of a browser environment, so browser-specific features like Service Workers and Cache API are unavailable.
+
+For a complete reference, see the [WHATWG Fetch Standard](https://fetch.spec.whatwg.org/).