arm: cortex_m: update docs to reflect the lazy stacking functionality

Update the documentation for Lazy Stacking in Cortex-M, to
reflect the functionality changes (activate the lazy stacking
dynamically when building with MPU stack guards).

Make a note that the FP sharing mode is now default in Cortex-M.

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