arch: arm: cortex_m: Rephrase comment on IRQ decrement

Move and rephrase the comment to better explain reason why obtained
IRQ number is decremented.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

Author: de-nordic

arch/arm/core/cortex_m/isr_wrapper.c

@@ -68,9 +68,12 @@ void _isr_wrapper(void)
 #if defined(CONFIG_ARM_CUSTOM_INTERRUPT_CONTROLLER)
 	int32_t irq_number = z_soc_irq_get_active();
 #else
-	/* _sw_isr_table does not map the exceptions, only the interrupts. */
 	int32_t irq_number = __get_IPSR();
 #endif
+	/* _sw_isr_table does not map the core system exceptions,
+	 * which take first 16 interrupt numbers, only the external
+	 * interrupts.
+	 */
 	irq_number -= 16;
 
 	struct _isr_table_entry *entry = &_sw_isr_table[irq_number];