Update notes for service default API version

Author: David R. Williamson

iothub/service/src/ClientApiVersionHelper.cs

@@ -4,16 +4,21 @@
 namespace Microsoft.Azure.Devices
 {
     /// <summary>
-    /// Holds the API version numbers required in data-plane calls to the service
+    /// Holds the API version numbers required in data-plane calls to the service for <see cref="RegistryManager"/>,
+    /// <see cref="ServiceClient"/>, and <see cref="JobClient"/>.
     /// </summary>
     internal class ClientApiVersionHelper
     {
         private const string ApiVersionQueryPrefix = "api-version=";
         private const string ApiVersionDefault = "2021-04-12";
 
         /// <summary>
-        /// The default API version to use for all data-plane service calls
+        /// The default API version query string parameter to use for all data-plane service calls in <see cref="RegistryManager"/>,
+        /// <see cref="ServiceClient"/>, and <see cref="JobClient"/>.
         /// </summary>
         public const string ApiVersionQueryString = ApiVersionQueryPrefix + ApiVersionDefault;
+
+        // For DigitalTwinClient which has an autorest-generated protocol layer, the API version is specified at generation time.
+        // To update that API version, follow the instructions at ./DigitalTwin/readme.md.
     }
 }