arch: arm: enhance internal function documentation
arch: arm: enhance internal function documentation

This commit enhaces the documentation of internal functions
in arm_mpu.c by explicitly stating that the caller needs to
ensure the validity of the supplied MPU region index. The
warning is required as these functions modify the ARM MPU_RNR
register, without checking themselves the validity of the
provided region number.

Signed-off-by: Ioannis Glaropoulos <5921cc8bab7e1d4329f52fd8f6268f9692e3de80@nordicsemi.no>

finikorg/zephyr,ldts/zephyr,ldts/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,explora26/zephyr,nashif/zephyr,mbolivar/zephyr,zephyriot/zephyr,finikorg/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,punitvara/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,kraj/zephyr,mbolivar/zephyr,punitvara/zephyr,zephyriot/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,punitvara/zephyr,kraj/zephyr,explora26/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,galak/zephyr,ldts/zephyr,mbolivar/zephyr,Vudentz/zephyr,nashif/zephyr,mbolivar/zephyr,galak/zephyr,nashif/zephyr,kraj/zephyr,ldts/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,Vudentz/zephyr,zephyriot/zephyr,zephyriot/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,punitvara/zephyr