Update IdentityServiceCollectionExtensions.cs docs (#1962)

src/Core/IdentityServiceCollectionExtensions.cs

@@ -13,7 +13,8 @@ namespace Microsoft.Extensions.DependencyInjection
     public static class IdentityServiceCollectionExtensions
     {
         /// <summary>
-        /// Adds and configures the identity system for the specified User and Role types.
+        /// Adds and configures the identity system for the specified User type. Role services are not added 
+        /// by default but can be added with <see cref="IdentityBuilder.AddRoles{TRole}"/>.
         /// </summary>
         /// <typeparam name="TUser">The type representing a User in the system.</typeparam>
         /// <param name="services">The services available in the application.</param>
@@ -22,7 +23,8 @@ namespace Microsoft.Extensions.DependencyInjection
             => services.AddIdentityCore<TUser>(o => { });
 
         /// <summary>
-        /// Adds and configures the identity system for the specified User and Role types.
+        /// Adds and configures the identity system for the specified User type. Role services are not added by default 
+        /// but can be added with <see cref="IdentityBuilder.AddRoles{TRole}"/>.
         /// </summary>
         /// <typeparam name="TUser">The type representing a User in the system.</typeparam>
         /// <param name="services">The services available in the application.</param>
@@ -52,4 +54,4 @@ namespace Microsoft.Extensions.DependencyInjection
             return new IdentityBuilder(typeof(TUser), services);
         }
     }
-}
+}