x86/irq: describe how the interrupt CPU movement works

x86/irq: describe how the interrupt CPU movement works

The logic to move interrupts across CPUs is complex, attempt to provide a
comment that describes the expected behavior so users of the interrupt system
have more context about the usage of the arch_irq_desc structure fields.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>