Document counterintuitive AddHostedService behavior (dotnet#9141)

* Document counterintuitive AddHostedService behavior

See dotnet/runtime#52747 for discussion about this issue.

* Update ServiceCollectionHostedServiceExtensions.xml

* Fix link in ServiceCollectionHostedServiceExtensions.xml

* Apply suggestions from code review

* Apply suggestions from code review

Co-authored-by: Genevieve Warren <24882762+gewarren@users.noreply.github.com>

* Fix CDATA parse error

---------

Co-authored-by: Buyaa Namnan <buyankhishig.namnan@microsoft.com>
Co-authored-by: Genevieve Warren <24882762+gewarren@users.noreply.github.com>

Author: 3 people

xml/Microsoft.Extensions.DependencyInjection/ServiceCollectionHostedServiceExtensions.xml

@@ -69,7 +69,12 @@
         <param name="services">The <see cref="T:Microsoft.Extensions.DependencyInjection.IServiceCollection" /> to register with.</param>
         <summary>Add an <see cref="T:Microsoft.Extensions.Hosting.IHostedService" /> registration for the given type.</summary>
         <returns>The original <see cref="T:Microsoft.Extensions.DependencyInjection.IServiceCollection" />.</returns>
-        <remarks>To be added.</remarks>
+        <remarks>
+          <format type="text/markdown"><![CDATA[
+> [!NOTE]
+> You shouldn't call this method multiple times for the same class. Doing so will only register the first HostedService you provide. To add multiple HostedServices, use <xref:Microsoft.Extensions.DependencyInjection.ServiceCollectionServiceExtensions.AddSingleton%60%601(Microsoft.Extensions.DependencyInjection.IServiceCollection)> for each implementation of <xref:Microsoft.Extensions.Hosting.IHostedService> explicitly.
+          ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="AddHostedService&lt;THostedService&gt;">
@@ -110,7 +115,12 @@
         <param name="implementationFactory">A factory to create new instances of the service implementation.</param>
         <summary>Add an <see cref="T:Microsoft.Extensions.Hosting.IHostedService" /> registration for the given type.</summary>
         <returns>The original <see cref="T:Microsoft.Extensions.DependencyInjection.IServiceCollection" />.</returns>
-        <remarks>To be added.</remarks>
+        <remarks>
+          <format type="text/markdown"><![CDATA[
+> [!NOTE]
+> You shouldn't call this method multiple times for the same class. Doing so will only register the first HostedService you provide. To add multiple HostedServices, use <xref:Microsoft.Extensions.DependencyInjection.ServiceCollectionServiceExtensions.AddSingleton%60%601(Microsoft.Extensions.DependencyInjection.IServiceCollection,System.Func{System.IServiceProvider,%60%600})> for each implementation of <xref:Microsoft.Extensions.Hosting.IHostedService> explicitly.
+          ]]></format>
+         </remarks>
       </Docs>
     </Member>
   </Members>