arch: arm: improve documentation for ARMv8-M fixed region setup

Improve the documentation of the ARMv8-M MPU convenience macros
for setting up MPU regions at boot time, stressing that the
macros intend to be used for non-overlapping, fixed MPU regions.

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