arch: arm: aarch32: fix documentation in z_irq_spurious definition

Correct documentation note in z_irq_spurious() definition,
stressing that the function is installed in _sw_isr_table
entries at boot time (which may be or not be used for
dynamic interrupts).

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