Update lib/src/models/google_play_services_availability.dart

Co-authored-by: Maurits van Beusekom <[email protected]>

Author: JDDV

lib/src/models/google_play_services_availability.dart

@@ -7,7 +7,9 @@ class GooglePlayServicesAvailability {
   /// Returns the value of the status as a string of the given status integer
   factory GooglePlayServicesAvailability.byValue(int value) => values[value];
 
-  ///Only used for testing purposes
+  /// Creates an instance of the [GoogleApiAvailability] class. This 
+  /// constructor is exposed for testing purposes only and should not be used 
+  /// by clients of the plugin as it may break or change at any time. 
   @visibleForTesting
   const GooglePlayServicesAvailability.private(this.value);