Update documentation for AddHostedService. (#123658)

Fixes #53831.

Author: cincuranet

src/libraries/Microsoft.Extensions.Hosting.Abstractions/src/ServiceCollectionHostedServiceExtensions.cs

@@ -16,6 +16,19 @@ public static class ServiceCollectionHostedServiceExtensions
         /// <summary>
         /// Add an <see cref="IHostedService"/> registration for the given type.
         /// </summary>
+        /// <remarks>
+        /// Note that this creates registration for <see cref="IHostedService"/> specifically. Not for the actual <c>THostedService</c> type.
+        /// If you want to register the actual type, you must do so separately.
+        /// </remarks>
+        /// <example>
+        /// <para>
+        /// The following code shows how to register a hosted service while also registering the actual <c>THostedService</c> type.
+        /// </para>
+        /// <code language="csharp">
+        /// services.AddSingleton&lt;SomeService&gt;();
+        /// services.AddHostedService(sp => sp.GetRequiredService&lt;SomeService&gt;());
+        /// </code>
+        /// </example>
         /// <typeparam name="THostedService">An <see cref="IHostedService"/> to register.</typeparam>
         /// <param name="services">The <see cref="IServiceCollection"/> to register with.</param>
         /// <returns>The original <see cref="IServiceCollection"/>.</returns>
@@ -30,6 +43,19 @@ public static class ServiceCollectionHostedServiceExtensions
         /// <summary>
         /// Add an <see cref="IHostedService"/> registration for the given type.
         /// </summary>
+        /// <remarks>
+        /// Note that this creates registration for <see cref="IHostedService"/> specifically. Not for the actual <c>THostedService</c> type.
+        /// If you want to register the actual type, you must do so separately.
+        /// </remarks>
+        /// <example>
+        /// <para>
+        /// The following code shows how to register a hosted service while also registering the actual <c>THostedService</c> type.
+        /// </para>
+        /// <code language="csharp">
+        /// services.AddSingleton&lt;SomeService&gt;(implementationFactory);
+        /// services.AddHostedService(sp => sp.GetRequiredService&lt;SomeService&gt;());
+        /// </code>
+        /// </example>
         /// <typeparam name="THostedService">An <see cref="IHostedService"/> to register.</typeparam>
         /// <param name="services">The <see cref="IServiceCollection"/> to register with.</param>
         /// <param name="implementationFactory">A factory to create new instances of the service implementation.</param>