PCI/MSI: Update MSI/MSI-X bits in PCIEBUS-HOWTO

Update the MSI/MSI-X bits in PCIEBUS-HOWTO.  Stop talking about low-level
details that mention deprecated APIs and concentrate on what service
drivers should do and why.

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