[flang][docs] Update release notes for OpenMP (#147988)

Co-authored-by: Kiran Chandramohan <kiranchandramohan@gmail.com>

Author: tblah

flang/docs/ReleaseNotes.md

@@ -25,6 +25,8 @@ page](https://llvm.org/releases/).
 ## Major New Features
 
 * Initial support for VOLATILE variables and procedure interface arguments has been added.
+* OpenMP support is stable and no longer considered experimental. All of OpenMP 3.1 is
+  supported, along with much of OpenMP 4.0 and some parts of later standards.
 
 ## Bug Fixes