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

Documentation/PCI/msi-howto.rst

@@ -285,3 +285,13 @@ to bridges between the PCI root and the device, MSIs are disabled.
 It is also worth checking the device driver to see whether it supports MSIs.
 For example, it may contain calls to pci_alloc_irq_vectors() with the
 PCI_IRQ_MSI or PCI_IRQ_MSIX flags.
+
+
+List of device drivers MSI(-X) APIs
+===================================
+
+The PCI/MSI subystem has a dedicated C file for its exported device driver
+APIs — `drivers/pci/msi/api.c`. The following functions are exported:
+
+.. kernel-doc:: drivers/pci/msi/api.c
+   :export: