Update service-fabric-cluster-capacity.md

Add mention that downgrading durability levels is unsupported per ServiceFabric engineering team. Ran into an issue when downgrading from Gold to Silver that busted our SF cluster that prevented us from doing any VMSS operations until roughly 20% of the instances were deleted. Root cause was stated to be a customer self-inflicted issue by downgrading durability levels.

Author: Jack Williams

articles/service-fabric/service-fabric-cluster-capacity.md

@@ -117,6 +117,7 @@ Follow these recommendations for managing node types with Silver or Gold durabil
 Within certain constraints, node type durability level can be adjusted:
 
 * Node types with durability levels of Silver or Gold can't be downgraded to Bronze.
+* Downgrading node types with durability level of Gold to Silver is not supported.
 * Upgrading from Bronze to Silver or Gold can take a few hours.
 * When changing durability level, be sure to update it in both the Service Fabric extension configuration in your virtual machine scale set resource and in the node type definition in your Service Fabric cluster resource. These values must match.
 
@@ -202,4 +203,4 @@ For more on cluster planning, see:
 * [Disaster recovery planning](service-fabric-disaster-recovery.md)
 
 <!--Image references-->
-[SystemServices]: ./media/service-fabric-cluster-capacity/SystemServices.png
+[SystemServices]: ./media/service-fabric-cluster-capacity/SystemServices.png