PCI/MSI: Document the various ways of ending up with NO_MSI

We have now three ways of ending up with NO_MSI being set.
Document them.

Link: https://lore.kernel.org/r/20210330151145.997953-14-maz@kernel.org


Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>