This commit corrects and improves the documentation for the
convenience macro _ARCH_THREAD_STACK_SIZEOF(sym). It stresses
that the returned size is guaranteed to match the amount of
stack that is available for the thread, that is, excluding any
areas not directly usable, e.g. a Stack Guard.
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>