Update overview with failover details

Update Azure Resource Manager overview with further resiliency details during region, zone, and instance unavailability

Author: jennyhunter-msft

articles/azure-resource-manager/management/overview.md

@@ -104,9 +104,9 @@ There are some important factors to consider when defining your resource group:
 
 The Azure Resource Manager service is designed for resiliency and continuous availability. Resource Manager and control plane operations (requests sent to `management.azure.com`) in the REST API are:
 
-* Distributed across regions. Some services are regional.
+* Distributed across regions. Azure Resource Manager has a separate instance in each region of Azure, meaning that a failure of the Azure Resource Manager instance in one region will not impact the availability of Azure Resource Manager or other Azure services in another region. Although Azure Resource Manager is distributed across regions, some services are regional. This distinction means that while the initial handling of the control plane operation is resilient, the request may be susceptible to regional outages when forwarded to the service.
 
-* Distributed across Availability Zones (and regions) in locations that have multiple Availability Zones.
+* Distributed across Availability Zones (and regions) in locations that have multiple Availability Zones. This ensures that when a region loses a single zone, Azure Resource Manager can either failover to another zone or to another region to continue to operate. In the case of multi-zone loss, Azure Resource Manager can failover to another region to continue to provide control plane capability for resources in the unavailable region.
 
 * Not dependent on a single logical data center.