PCI: endpoint: Add missing documentation about the MSI/MSI-X range

PCI: endpoint: Add missing documentation about the MSI/MSI-X range

Both pci_epc_raise_irq() and pci_epc_map_msi_irq() APIs expect the
MSI/MSI-X vectors to start from 1 but it is not documented. Add the range
info to the kdoc of the APIs to make it clear.

Link: https://lore.kernel.org/r/20230602114756.36586-2-manivannan.sadhasivam@linaro.org
Fixes: 5e8cb4033807 ("PCI: endpoint: Add EP core layer to enable EP controller and EP functions")
Fixes: 87d5972e476f ("PCI: endpoint: Add pci_epc_ops to map MSI IRQ")
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>