x86, ioapic: Document another case when level irq is seen as an edge

In the case when cpu goes offline, fixup_irqs() will forward any
unhandled interrupt on the offlined cpu to the new cpu
destination that is handling the corresponding interrupt. This
interrupt forwarding is done via IPI's. Hence, in this case also
level-triggered io-apic interrupt will be seen as an edge
interrupt in the cpu's APIC IRR.

Document this scenario in the code which handles this case by doing
an explicit EOI to the io-apic to clear remote IRR of the io-apic RTE.

Requested-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Cc: ebiederm@xmission.com
Cc: garyhade@us.ibm.com
LKML-Reference: <20091201233335.143970505@sbs-t61.sc.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>