Update connectivity notes for private endpoint resources

Added note about potential connectivity issues during Gateway SKU upgrades.

Author: vishalme

articles/expressroute/expressroute-about-virtual-network-gateways.md

@@ -124,6 +124,7 @@ The ExpressRoute virtual network gateway facilitates connectivity to private end
 > [!IMPORTANT]
 > * The throughput and control plane capacity for connectivity to private endpoint resources might be reduced by half compared to connectivity to non-private endpoint resources.
 > * During a maintenance period, you might experience intermittent connectivity problems to private endpoint resources.
+> * During Gateway SKU upgrade, you might experience intermittent connectivity problems to private endpoint resources.
 > * You need to ensure that on-premises configuration, including router and firewall settings, are correctly set up to ensure that packets for the IP 5-tuple transits use a single next hop (Microsoft Enterprise Edge router) unless there's a maintenance event. If your on-premises firewall or router configuration is causing the same IP 5-tuple to frequently switch next hops, you experience connectivity problems.
 > * Ensure that [network policies](../private-link/disable-private-endpoint-network-policy.md) (at a minimum, for UDR support) are enabled on the subnet(s) where private endpoints are deployed