PCI/MSI: Add pci_enable_msi() documentation back

We deprecated pci_enable_msi() in 302a2523 ("PCI/MSI: Add
pci_enable_msi_range() and pci_enable_msix_range()").

But we changed our minds after noticing that:

  - pci_enable_msi() doesn't have confusing return values like
    pci_enable_msi_block() and pci_enable_msix() did, and

  - pci_enable_msi() has a hundred or so callers that we don't want to
    change.

This adds back the pci_enable_msi() documentation.

[bhelgaas: changelog]
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>