PCI: Expand documentation for pci_add_dma_alias()

Seeing there's been some confusion about the use of pci_add_dma_alias(),
expand the comment to describe why it must be called early and how
early it must be called.

Also, expand on the purpose of this function and common reasons it would
be used.

[The comment was reworded to some extent by Alex Williamson]

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Doug Meyer <dmeyer@gigaio.com>