add missing param comment (#380)

Author: zhiyuanliang-ms

src/Microsoft.FeatureManagement/IVariantFeatureManager.cs

@@ -16,6 +16,7 @@ public interface IVariantFeatureManager
         /// <summary>
         /// Retrieves a list of feature names registered in the feature manager.
         /// </summary>
+        /// <param name="cancellationToken">The cancellation token to cancel the operation.</param>
         /// <returns>An enumerator which provides asynchronous iteration over the feature names registered in the feature manager.</returns>
         IAsyncEnumerable<string> GetFeatureNamesAsync(CancellationToken cancellationToken);