arch: arm: update doc for 8-byte stack alignment option for Cortex-M
arch: arm: update doc for 8-byte stack alignment option for Cortex-M

This commit updates the documentation of Kconfig option:
STACK_ALIGN_DOUBLE_WORD for Cortex-M microprocessors, stating
that the option is used in ARMv7-M MCUs to enforce 8-byte
stack alignment upon exception entry.

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

finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,ldts/zephyr,ldts/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,ldts/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,explora26/zephyr,explora26/zephyr,explora26/zephyr,ldts/zephyr,ldts/zephyr,nashif/zephyr,explora26/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr