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 <Ioannis.Glaropoulos@nordicsemi.no>