Add sponsorship credit note for Reserved Instances

Clarify that Azure Sponsorship credits cannot be used to purchase
Reserved Instances, even though reservations apply to eligible
resources in Sponsorship-backed subscriptions.

Co-authored-by: Copilot <[email protected]>

Author: pri-mittal

articles/cost-management-billing/reservations/prepare-buy-reservation.md

@@ -67,6 +67,9 @@ Reservation discounts apply to the following eligible subscriptions and offer ty
 - CSP subscriptions
 - Microsoft Azure Sponsorship subscriptions (offer number: MS-AZR-0036P)
 
+> [!NOTE]
+> Reservations may be applied to eligible resources running in Sponsorship-backed subscriptions; however, Azure Sponsorship credits cannot be used to purchase Reserved Instances.
+
 Resources that run in a subscription with other offer types don't receive the reservation discount.
 
 Some Azure services do not support Reservation (RI) discounts, so it's important to check eligibility before making a purchase to avoid unnecessary costs. For example: Azure SQL Serverless and Azure Cosmos DB serverless