documentation: add smddp allreduce improvements

Author: mchoi8739

doc/api/training/smd_data_parallel_release_notes/smd_data_parallel_change_log.rst

@@ -15,8 +15,11 @@ SageMaker Distributed Data Parallel 1.6.0 Release Notes
 **New Features**
 
 * New SMDDP Collectives support for the SageMaker model parallelism library’s sharded data parallelism operating AllGather. For more information, see `Sharded data parallelism with SMDDP Collectives <https://docs.aws.amazon.com/sagemaker/latest/dg/model-parallel-extended-features-pytorch-sharded-data-parallelism.html#model-parallel-extended-features-pytorch-sharded-data-parallelism-smddp-collectives>`_ in the Amazon SageMaker Developer Guide.
-* Added support for Amazon EC2 ml.p4de.24xlarge instances
+* Added support for Amazon EC2 ml.p4de.24xlarge instances.
 
+**Improvements*
+
+* Improved general performance of the SMDDP AllReduce collective communication operation.
 
 **Migration to AWS Deep Learning Containers**