Help: Clarify release note for IntelLLVM compiler support

Update the release note added by commit b9f945eccb (Help: Add release
note for IntelLLVM compiler support, 2021-01-29) to distinguish between
the Intel oneAPI NextGen and Classic compilers.

Co-Author: William R. Dieter <[email protected]>
Suggested-by: Michael Hirsch, Ph.D <[email protected]>

Help/release/dev/intel-llvm-compilers.rst

@@ -1,6 +1,19 @@
 intel-llvm-compilers
 --------------------
 
-* The Intel oneAPI Compilers are now supported with compiler id ``IntelLLVM``.
-  Support is available for the C, C++, and Fortran compilers on Linux, and
-  for the C and C++ compilers on Windows.
+* The Intel oneAPI NextGen LLVM compilers are now supported with
+  compiler id ``IntelLLVM``:
+
+  * The ``icx``/``icpx`` C/C++ compilers on Linux, and the ``icx``
+    C/C++ compiler on Windows, are fully supported as of oneAPI 2021.1.
+
+  * The ``ifx`` Fortran compiler on Linux is partially supported.
+    As of oneAPI 2021.1, ``ifx`` does not define several identification
+    macros, so CMake identifies it as the classic ``Intel`` compiler.
+    This works in many cases because ``ifx`` accepts the same command line
+    parameters as ``ifort``.
+
+  * The ``ifx`` Fortran compiler on Windows is not yet supported.
+
+  The Intel oneAPI Classic compilers (``icc``, ``icpc``, and ``ifort``)
+  continue to be supported with compiler id ``Intel``.