Documentation: PCI: Add background on Boot Interrupts

Improve understanding of the PCI quirks for this legacy PCI interrupt
behavior to the benefit of developers and users alike.

Link: https://lore.kernel.org/r/20200220192930.64820-3-sean.v.kelley@linux.intel.com


Signed-off-by: Sean V Kelley <sean.v.kelley@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>