doc: Update on switch to CMSIS_6

Add a note to announce the switch to CMSIS_6 for Cortex-M arch.

Signed-off-by: Sudan Landge <[email protected]>

Author: wearyzen

doc/hardware/arch/arm_cortex_m.rst

@@ -632,8 +632,16 @@ script as well.
 CMSIS
 *****
 
-Cortex-M CMSIS headers are hosted in a standalone module repository:
-`zephyrproject-rtos/cmsis <https://github.com/zephyrproject-rtos/cmsis>`_.
+Cortex-M CMSIS headers are provided through standalone module repositories:
+
+- **CMSIS 5**: `zephyrproject-rtos/cmsis <https://github.com/zephyrproject-rtos/cmsis>`_
+- **CMSIS 6**: `zephyrproject-rtos/CMSIS_6 <https://github.com/zephyrproject-rtos/CMSIS_6>`_
+
+Zephyr has begun transitioning to **CMSIS 6** as the default source for Cortex-M core headers.
+However, at present, Zephyr includes headers from **both** the CMSIS 6 and legacy CMSIS 5 modules.
+
+The legacy CMSIS 5 headers remain available primarily for compatibility with vendor HALs, while all
+new architecture-level development should use **CMSIS 6** headers whenever possible.
 
 :kconfig:option:`CONFIG_CPU_CORTEX_M` selects :kconfig:option:`CONFIG_HAS_CMSIS_CORE` to signify that
 CMSIS headers are available for all supported Cortex-M variants.

doc/releases/release-notes-4.2.rst

@@ -771,3 +771,9 @@ Other notable changes
 
 * Updated TF-M to version 2.1.2 (from 2.1.1). The release notes can be found at:
   https://trustedfirmware-m.readthedocs.io/en/tf-mv2.1.2/releases/2.1.2.html
+
+* As of commit f726cb5123eda12feb8746e3acb34a1f7d7b0340, Zephyr now uses the ``CMSIS_6`` module
+  (instead of ``cmsis`` which was CMSIS 5.9.0) for Cortex-M architectures.
+  Other modules still relying on the older ``cmsis`` module are encouraged to migrate to ``CMSIS_6``.
+  The path to the ``CMSIS_6`` module can be accessed via the ``CONFIG_ZEPHYR_CMSIS_6_MODULE_DIR`` configuration.
+  Note: Zephyr will continue using the older ``cmsis`` module for Cortex-A and Cortex-R targets.