Add line item discount limits to Add discount to checkout endpoint

reference/checkouts.v3.yml

@@ -1608,7 +1608,10 @@ paths:
 
         This discount only applies to `line_items`. When you call this API, you clear out all existing discounts applied to line items, including product and order-based discounts.
 
-        This endpoint splits the discount between line items based on the item value.
+        **Note:**
+        This endpoint splits the discount between line items based on the item value. 
+
+        You'll get an error message if you try to add a manual line item discount on a cart that has a promotion or coupon applied. The same limitations apply if you try to add a coupon or promotion to a cart that has an existing manual line item discount.
 
         Required Fields
         * discounted_amount