kernel: Clarify warning about no multithreading
kernel: Clarify warning about no multithreading

Clarify the warning in the help for CONFIG_MULTITHREADING to make it
clear that many things will break if this is set to 'n'.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

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