genirq / PM: describe IRQF_COND_SUSPEND

With certain restrictions it is possible for a wakeup device to share
an IRQ with an IRQF_NO_SUSPEND user, and the warnings introduced by
commit cab303be

 are spurious. The new
IRQF_COND_SUSPEND flag allows drivers to tell the core when these
restrictions are met, allowing spurious warnings to be silenced.

This patch documents how IRQF_COND_SUSPEND is expected to be used,
updating some of the text now made invalid by its addition.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>