Add PACBTI-M support to LLVM release notes.

Clang release note changes will be added once the last PACBTI-M command line
patch lands.

Reviewed By: vhscampos

Differential Revision: https://reviews.llvm.org/D115507

llvm/docs/ReleaseNotes.rst

@@ -83,6 +83,7 @@ Changes to the ARM Backend
 --------------------------
 
 * Added support for the Armv9-A, Armv9.1-A and Armv9.2-A architectures.
+* Added support for the Armv8.1-M PACBTI-M extension.
 
 Changes to the MIPS Target
 --------------------------