arch: arm: fatal: add documentation for z_do_kernel_oops()

Add some documentation for ARM-specific function
z_do_kernel_oops, stating clearly that it is only
invoked inside SVC context. We also comment on
the validity of the supplied ESF.

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