docs(core): Add explanation to Order.promotions field

packages/core/src/api/schema/type/order.type.graphql

@@ -14,6 +14,7 @@ type Order implements Node {
     "Order-level adjustments to the order total, such as discounts from promotions"
     adjustments: [Adjustment!]!
     couponCodes: [String!]!
+    "Promotions applied to the order. Only gets populated after the payment process has completed."
     promotions: [Promotion!]!
     payments: [Payment!]
     fulfillments: [Fulfillment!]