arch: arm: aarch32: cortex_m: improve documentation of z_arm_reset

We enhance the documentation of z_arm_reset, stressing that
the function may either be loaded by the processor coming
out of reset, or by another image, e.g. a bootloader. We
also specify what is required at minimum when executing the
reset function.

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