[Service Bus] Admin client performance note (Azure#39717)

* [Service Bus] Admin client performance note

The focus of these changes is to add a note about the performance characteristics
of the `ServiceBusAdministrationClient` with guidance to avoid using in
performance-critical scenarios.

* Update sdk/servicebus/Azure.Messaging.ServiceBus/src/Administration/ServiceBusAdministrationClient.cs

Author: jsquire

sdk/servicebus/Azure.Messaging.ServiceBus/src/Administration/ServiceBusAdministrationClient.cs

@@ -15,6 +15,11 @@ namespace Azure.Messaging.ServiceBus.Administration
     /// The <see cref="ServiceBusAdministrationClient"/> is the client through which all Service Bus
     /// entities can be created, updated, fetched, and deleted.
     /// </summary>
+    /// <remarks>
+    /// The <see cref="ServiceBusAdministrationClient"/> operates against an entity management endpoint
+    /// without performance guarantees.  It is not recommended for use in performance-critical
+    /// scenarios.
+    /// </remarks>
     public class ServiceBusAdministrationClient
     {
         private readonly string _fullyQualifiedNamespace;