Add info on PACBTI-M to the Clang release notes

[amilendra]

LGTM.

clang/docs/ReleaseNotes.rst

- Configuration file syntax extended with ``<CFGDIR>`` token. This expands to
   the base path of the current config file. See :ref:`configuration-files` for
   details.
 
 New Compiler Flags
 ------------------
 
 - Clang plugin arguments can now be passed through the compiler driver via
   ``-fplugin-arg-pluginname-arg``, similar to GCC's ``-fplugin-arg``.
+- The ``-mno-bti-at-return-twice`` flag will make sure a BTI instruction won't
+  be added after a setjmp or possible other return-twice construct (ARM backend
+  only).
 
 Deprecated Compiler Flags
 -------------------------
 
 - -Wweak-template-vtables has been deprecated and no longer has any effect. The
   flag will be removed in the next release.
 
 Modified Compiler Flags
- The -mtune flag is no longer ignored for AArch64. It is now possible to
   "-mcpu=generic -mtune=cortex-a57" will not enable any Cortex-A57 specific
   architecture features, but will enable certain optimizations specific to
   Cortex-A57 CPUs and enable the use of a more accurate scheduling model.
 
 - The --aarch64-none-elf target now uses the BareMetal driver rather than the
   GNU driver. Programs that depend on clang invoking GCC as the linker driver
   should use GCC as the linker in the build system.
 
+- The ``-mbranch-protection`` flag will now also work for the ARM backend.
+
+- The ``attribute((target("branch-protection=...)))`` attributes will now also
+  work for the ARM backend.
+
 Floating Point Support in Clang
 -------------------------------
 - The default setting of FP contraction (FMA) is now -ffp-contract=on (for
   languages other than CUDA/HIP) even when optimization is off. Previously,
   the default behavior was equivalent to -ffp-contract=off (-ffp-contract
   was not set).
   Related to this, the switch -ffp-model=precise now implies -ffp-contract=on
   rather than -ffp-contract=fast, and the documentation of these features has