PCI: Document /sys/bus/pci/devices/.../irq

PCI: Document /sys/bus/pci/devices/.../irq

Document /sys/bus/pci/devices/.../irq.

This file contains the IRQ of the INTx interrupt (or zero if the device
doesn't support INTx interrupts).

If the device has enabled MSI (not MSI-X), it contains the first MSI IRQ
instead.  This is a historical mistake because devices may support several
MSI or MSI-X vectors, and this file can't contain them all.  But we
preserve this behavior to avoid breaking userspace.

[bhelgaas: commit log]
Link: https://lore.kernel.org/r/20210825102636.52757-2-21cnbao@gmail.com
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>