[Hexagon] Add release note for LLVM 16

Krzysztof Parzyszek · tstellar · commit c4392bcc1d5d · 2023-02-07T17:04:02.000-08:00
(cherry picked from commit 140bc41)
diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
@@ -166,7 +166,7 @@ Changes to the DirectX Backend
 Changes to the Hexagon Backend
 ------------------------------
 
-* ...
+* The Hexagon backend now support V71 and V73 ISA.
 
 Changes to the LoongArch Backend
 --------------------------------
