Documented AppBuilder.

And made `Start` method private: clients should use
`SetupWithoutStarting`.

src/Avalonia.Controls/AppBuilder.cs

@@ -1,27 +1,51 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
-using Avalonia.Input;
 using System;
 
 namespace Avalonia.Controls
 {
+    /// <summary>
+    /// Initializes up platform-specific services for an <see cref="Application"/>.
+    /// </summary>
     public class AppBuilder
     {
+        /// <summary>
+        /// Gets or sets the <see cref="Application"/> instance being initialized.
+        /// </summary>
         public Application Instance { get; set; }
 
+        /// <summary>
+        /// Gets or sets a method to call the initialize the windowing subsystem.
+        /// </summary>
         public Action WindowingSubsystem { get; set; }
 
+        /// <summary>
+        /// Gets or sets a method to call the initialize the windowing subsystem.
+        /// </summary>
         public Action RenderingSubsystem { get; set; }
 
+        /// <summary>
+        /// Gets or sets a method to call before <see cref="Start{TMainWindow}"/> is called on the
+        /// <see cref="Application"/>.
+        /// </summary>
         public Action<AppBuilder> BeforeStartCallback { get; set; }
 
+        /// <summary>
+        /// Begin configuring an <see cref="Application"/>.
+        /// </summary>
+        /// <typeparam name="TApp">The subclass of <see cref="Application"/> to configure.</typeparam>
+        /// <returns>An <see cref="AppBuilder"/> instance.</returns>
         public static AppBuilder Configure<TApp>()
             where TApp : Application, new()
         {
             return Configure(new TApp());
         }
 
+        /// <summary>
+        /// Begin configuring an <see cref="Application"/>.
+        /// </summary>
+        /// <returns>An <see cref="AppBuilder"/> instance.</returns>
         public static AppBuilder Configure(Application app)
         {
             AvaloniaLocator.CurrentMutable.BindToSelf(app);
@@ -32,12 +56,22 @@ namespace Avalonia.Controls
             };
         }
 
+        /// <summary>
+        /// Registers a callback to call before <see cref="Start{TMainWindow}"/> is called on the
+        /// <see cref="Application"/>.
+        /// </summary>
+        /// <param name="callback">The callback.</param>
+        /// <returns>An <see cref="AppBuilder"/> instance.</returns>
         public AppBuilder BeforeStarting(Action<AppBuilder> callback)
         {
             BeforeStartCallback = callback;
             return this;
         }
 
+        /// <summary>
+        /// Starts the application with an instance of <typeparamref name="TMainWindow"/>.
+        /// </summary>
+        /// <typeparam name="TMainWindow">The window type.</typeparam>
         public void Start<TMainWindow>()
             where TMainWindow : Window, new()
         {
@@ -49,25 +83,42 @@ namespace Avalonia.Controls
             Instance.Run(window);
         }
 
+        /// <summary>
+        /// Sets up the platform-specific services for the application, but does not run it.
+        /// </summary>
+        /// <returns></returns>
         public AppBuilder SetupWithoutStarting()
         {
             Setup();
             return this;
         }
 
+        /// <summary>
+        /// Specifies a windowing subsystem to use.
+        /// </summary>
+        /// <param name="initializer">The method to call to initialize the windowing subsystem.</param>
+        /// <returns>An <see cref="AppBuilder"/> instance.</returns>
         public AppBuilder WithWindowingSubsystem(Action initializer)
         {
             WindowingSubsystem = initializer;
             return this;
         }
 
+        /// <summary>
+        /// Specifies a rendering subsystem to use.
+        /// </summary>
+        /// <param name="initializer">The method to call to initialize the rendering subsystem.</param>
+        /// <returns>An <see cref="AppBuilder"/> instance.</returns>
         public AppBuilder WithRenderingSubsystem(Action initializer)
         {
             RenderingSubsystem = initializer;
             return this;
         }
 
-        public void Setup()
+        /// <summary>
+        /// Sets up the platform-speciic services for the <see cref="Application"/>.
+        /// </summary>
+        private void Setup()
         {
             if (Instance == null)
             {
@@ -83,6 +134,7 @@ namespace Avalonia.Controls
             {
                 throw new InvalidOperationException("No rendering system configured.");
             }
+
             Instance.RegisterServices();
             WindowingSubsystem();
             RenderingSubsystem();

src/Avalonia.DesignerSupport/DesignerAssist.cs

@@ -62,7 +62,7 @@ namespace Avalonia.DesignerSupport
             AppBuilder.Configure(app == null ? new DesignerApp() : (Application) Activator.CreateInstance(app.AsType()))
                 .WithWindowingSubsystem(Application.InitializeWin32Subsystem)
                 .WithRenderingSubsystem(() => { })
-                .Setup();
+                .SetupWithoutStarting();
         }
 
         private static void SetScalingFactor(double factor)