docs: add more clarification on updating a cart's customer

www/apps/resources/app/storefront-development/cart/update/page.mdx

@@ -53,6 +53,11 @@ The Update Cart API route accepts a `region_id` request body parameter, whose va
 
 If a guest customer logs in, you must update the cart to be associated with the logged-in customer.
 
+The request to update the cart's associated customer must be authenticated:
+
+- If the customer is authenticated by a session, include `credentials: include` in the fetch requests.
+- If the customer is authenticated by a JWT token, include it in the Bearer Authorization header.
+
 <Note>
 
 You can't change the `customer_id` if the cart was associated with a customer previously.