docs: update guide on return urls (#206)

Author: tomasvotava

docs/how-to-guides/state-return-url.md

@@ -2,7 +2,8 @@
 
 State is useful if you want the server to return something back to you to help you understand in what
 context the authentication was initiated. It is mostly used to store the url you want your user to be redirected
-to after successful login. You may use `.state` property to get the state returned from the server.
+to after successful login. You may use `.state` property to get the state returned from the server or access
+it from the `state` parameter in the callback function.
 
 Example:
 
@@ -19,9 +20,11 @@ async def google_login(return_url: str):
         # Send return_url to Google as a state so that Google knows to return it back to us
         return await google_sso.get_login_redirect(redirect_uri=request.url_for("google_callback"), state=return_url)
 
-async def google_callback(request: Request):
+async def google_callback(request: Request, state: str | None = None):
     async with google_sso:
         user = await google_sso.verify_and_process(request)
-        # google_sso.state now holds your return_url (https://example.com/welcome)
-        return RedirectResponse(google_sso.state)
+        if state is not None:
+            return RedirectResponse(state)
+        else:
+            return user
 ```