arch: arm: add explanatory comment for SPLIM_NS set functions

This commit adds an inline explanatory comment in the API
for setting the non-secure stack-pointer limit registers, to
stress that the API is only applicable to Mainline ARMv8-M MCUs.

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