Add addtional test and some comments

src/Microsoft.AspNetCore.Hosting/WebHostBuilder.cs

@@ -221,8 +221,8 @@ namespace Microsoft.AspNetCore.Hosting
                 configureLogging(_loggerFactory);
             }
 
-            services.AddSingleton(_loggerFactory);
+            services.AddSingleton(_loggerFactory); //The configured ILoggerFactory is added as a singleton here. AddLogging below will not add an additional one.
-            services.AddLogging();
+            services.AddLogging(); //This is required to add ILogger of T.
 
             services.AddTransient<IStartupLoader, StartupLoader>();
 

test/Microsoft.AspNetCore.Hosting.Tests/WebHostBuilderTests.cs

@@ -169,6 +169,26 @@ namespace Microsoft.AspNetCore.Hosting
             Assert.Same(loggerFactory, host.Services.GetService<ILoggerFactory>());
         }
 
+        [Fact]
+        public void MultipleConfigureLoggingInvokedInOrder()
+        {
+            var callCount = 0; //Verify ordering
+            var hostBuilder = new WebHostBuilder()
+                .ConfigureLogging(loggerFactory =>
+                {
+                    Assert.Equal(0, callCount++);
+                })
+                .ConfigureLogging(loggerFactory =>
+                {
+                    Assert.Equal(1, callCount++);
+                })
+                .UseServer(new TestServer())
+                .UseStartup<StartupNoServices>();
+
+            var host = (WebHost)hostBuilder.Build();
+            Assert.Equal(2, callCount);
+        }
+
         [Fact]
         public void DefaultHostingConfigurationDoesNotCaptureStartupErrors()
         {