add a comment explaining the ordering

src/libraries/Microsoft.Extensions.DependencyInjection/src/ServiceProvider.cs

@@ -32,6 +32,7 @@ public sealed class ServiceProvider : IServiceProvider, IDisposable, IAsyncDispo
 
         internal ServiceProvider(IEnumerable<ServiceDescriptor> serviceDescriptors, ServiceProviderOptions options)
         {
+            // note that Root needs to be set before calling GetEngine(), because the engine may need to access Root
             Root = new ServiceProviderEngineScope(this, isRootScope: true);
             _engine = GetEngine();
             _createServiceAccessor = CreateServiceAccessor;