Fix #4929 - Move IActionInvokerFactory from .Internal

Also added a wealth of doc comments to an area that's not currently super
well documented.

src/Microsoft.AspNetCore.Mvc.Abstractions/Abstractions/IActionInvoker.cs

@@ -5,8 +5,19 @@
 
 namespace Microsoft.AspNetCore.Mvc.Abstractions
 {
+    /// <summary>
+    /// Defines an interface for invoking an MVC action.
+    /// </summary>
+    /// <remarks>
+    /// An <see cref="IActionInvoker"/> is created for each request the MVC handles by querying the set of
+    /// <see cref="IActionInvokerProvider"/> instances. See <see cref="IActionInvokerProvider"/> for more information. 
+    /// </remarks>
     public interface IActionInvoker
     {
+        /// <summary>
+        /// Invokes an MVC action.
+        /// </summary>
+        /// <returns>A <see cref="Task"/> which will complete when action processing has completed.</returns>
         Task InvokeAsync();
     }
 }

src/Microsoft.AspNetCore.Mvc.Abstractions/Abstractions/IActionInvokerProvider.cs

@@ -3,6 +3,27 @@
 
 namespace Microsoft.AspNetCore.Mvc.Abstractions
 {
+    /// <summary>
+    /// Defines an interface for components that can create an <see cref="IActionInvoker"/> for the
+    /// current request.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// <see cref="IActionInvokerProvider"/> instances form a pipeline that results in the creation of an
+    /// <see cref="IActionInvoker"/>. The <see cref="IActionInvokerProvider"/> instances are ordered by
+    /// an ascending sort of the <see cref="Order"/>.  
+    /// </para>
+    /// <para>
+    /// To create an <see cref="IActionInvoker"/>, each provider has its <see cref="OnProvidersExecuting"/> method
+    /// called in sequence and given the same instance of <see cref="ActionInvokerProviderContext"/>. Then each
+    /// provider has its <see cref="OnProvidersExecuted"/> method called in the reverse order. The result is
+    /// the value of <see cref="ActionInvokerProviderContext.Result"/>.  
+    /// </para>
+    /// <para>
+    /// As providers are called in a predefined sequence, each provider has a chance to observe and decorate the
+    /// result of the providers that have already run. 
+    /// </para>
+    /// </remarks>
     public interface IActionInvokerProvider
     {
         /// <summary>
@@ -26,8 +47,16 @@ public interface IActionInvokerProvider
         /// </remarks>
         int Order { get; }
 
+        /// <summary>
+        /// Called to execute the provider. 
+        /// </summary>
+        /// <param name="context">The <see cref="ActionInvokerProviderContext"/>.</param>
         void OnProvidersExecuting(ActionInvokerProviderContext context);
 
+        /// <summary>
+        /// Called to execute the provider, after all <see cref="OnProvidersExecuting"/> methods have been called.
+        /// </summary>
+        /// <param name="context">The <see cref="ActionInvokerProviderContext"/>.</param>
         void OnProvidersExecuted(ActionInvokerProviderContext context);
     }
 }

src/Microsoft.AspNetCore.Mvc.Core/Infrastructure/IActionInvokerFactory.cs

@@ -0,0 +1,28 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Mvc.Abstractions;
+
+namespace Microsoft.AspNetCore.Mvc.Infrastructure
+{
+    /// <summary>
+    /// Defines an interface for creating an <see cref="IActionInvoker"/> for the current request. 
+    /// </summary>
+    /// <remarks>
+    /// The default <see cref="IActionInvokerFactory"/> implementation creates an <see cref="IActionInvoker"/> by
+    /// calling into each <see cref="IActionInvokerProvider"/>. See <see cref="IActionInvokerProvider"/> for more
+    /// details.
+    /// </remarks>
+    public interface IActionInvokerFactory
+    {
+        /// <summary>
+        /// Creates an <see cref="IActionInvoker"/> for the current request associated with
+        /// <paramref name="actionContext"/>. 
+        /// </summary>
+        /// <param name="actionContext">
+        /// The <see cref="ActionContext"/> associated with the current request.
+        /// </param>
+        /// <returns>An <see cref="IActionInvoker"/> or <c>null</c>.</returns>
+        IActionInvoker CreateInvoker(ActionContext actionContext);
+    }
+}