genirq: Expand doc for PENDING and REPLAY flags

Adding a bit more info about what the flags are used for may help future
code readers.

Signed-off-by: James Gowans <jgowans@amazon.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Liao Chang <liaochang1@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230608120021.3273400-2-jgowans@amazon.com