arch: arm: aarch32: cortex_m: add note in mem_manage_fault()

It is possible that MMFAR address is not written by the
Cortex-M core; this occurs when the stacking error is
not accompanied by a data access violation error (i.e.
when stack overflows due to the exception entry frame
stacking): z_check_thread_stack_fail() shall be able to
handle the case of 'mmfar' holding the -EINVAL value.

Add this node in mem_manage_fault() function to clarify
that it is valid for z_check_thread_stack_fail() to be
called with invalid mmfar address value.

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