PCI/MSI: Document pci_alloc_irq_vectors(), deprecate pci_enable_msi()

Document pci_alloc_irq_vectors() instead of the deprecated pci_enable_msi()
and pci_enable_msix() APIs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>