doc: Add MPI-4.0 partial compliance features

Signed-off-by: William Zhang <[email protected]>

Author: wckzhang

docs/release-notes/mpi.rst

@@ -31,6 +31,27 @@ For historical reference:
    * - MPI-3.1
      - Open MPI v2.0
 
+MPI-4.0 partial compliance
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In the Open MPI |ompi_series| series, only partial MPI-4.0 functionality is
+supported. This section contains a list of features added for the release.
+
+* Support for MPI Sessions has been added.
+* Added partitioned communication using persistent sends and persistent receives.
+* Added persistent collectives to the ``MPI_`` namespace (they were previously
+  available via the ``MPIX_ prefix``).
+* Added ``MPI_Isendrecv()`` and its variants.
+* Added support for ``MPI_Comm_idup_with_info()``.
+* Added support for ``MPI_Info_get_string()``.
+* Added support for ``initial_error_handler`` and the ``ERRORS_ABORT`` infrastructure.
+* Added error handling for “unbound” errors to ``MPI_COMM_SELF``.
+* Made ``MPI_Comm_get_info()``, ``MPI_File_get_info()``, and
+  ``MPI_Win_get_info()`` compliant to the standard.
+* Droped unknown/ignored info keys on communicators, files, and windows.
+* Initial implementations of ``MPI_COMM_TYPE_HW_GUIDED`` and
+  ``MPI_COMM_TYPE_HW_GUIDED`` added.
+
 Removed MPI APIs
 ----------------