Documentation: PCI: Add reference to PCI/MSI device driver APIs

All exported device-driver MSI APIs are now grouped in one place at
drivers/pci/msi/api.c with comprehensive kernel-docs added.

Reference these kernel-docs in the official PCI/MSI howto.

Signed-off-by: Ahmed S. Darwish <darwi@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20221111122015.397739421@linutronix.de