add note about app-only mutation in quickstart (saleor#1421)

docs/quickstart/api.mdx

@@ -657,6 +657,8 @@ subscription {
             If you forgot or lost a token from the previous step, you can create another one in the app settings.
 
             After executing the mutation, you should be able to see the order in the dashboard and a payload in the https://webhook.site/ with the order details provided in the subscription query.
+
+            Note: [`orderCreateFromCheckout`](/api-reference/orders/mutations/order-create-from-checkout) mutation requires the App Token. To transform the checkout on behalf of the customer, use [`checkoutComplete`](/api-reference/checkout/mutations/checkout-complete) mutation.
         </S.Details>
 
         <S.Code>