[Clang] Update ReleaseNotes with ThreadSafetyAnalysis changes (#155687)

Note that ACQUIRED_BEFORE(...) and ACQUIRED_AFTER(...) no longer require
-Wthread-safety-beta.

Follow-up from #152853.

Author: melver

clang/docs/ReleaseNotes.rst

@@ -270,6 +270,10 @@ Improvements to Clang's diagnostics
   decorated with the ``alloc_size`` attribute don't allocate enough space for
   the target pointer type.
 
+- The :doc:`ThreadSafetyAnalysis` attributes ``ACQUIRED_BEFORE(...)`` and
+  ``ACQUIRED_AFTER(...)`` have been moved to the stable feature set and no
+  longer require ``-Wthread-safety-beta`` to be used.
+
 Improvements to Clang's time-trace
 ----------------------------------