Help: Clarify release note for IntelLLVM compiler support

Update the release note added by commit b9f945ec (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 <william.r.dieter@intel.com>
Suggested-by: Michael Hirsch, Ph.D <michael@scivision.dev>
24 jobs for intel-llvm-compilers