MFC 232362,232363,232463,232553,232569,232571:

MFC 232362,232363,232463,232553,232569,232571:
- Sort function prototypes.
- Update the documentation on pci_get/set_powerstate().  These methods are
  not ACPI-specific at all, but deal with PCI power states.  Also,
  pci_set_powerstate() fails with EOPNOTSUPP if a request is made that the
  underlying device does not support rather than falling back to somehow
  setting D0.
- Document the following routines: pci_alloc_msi(), pci_alloc_msix(),
  pci_find_cap(), pci_get_max_read_req(), pci_get_vpd_ident(),
  pci_get_vpd_readonly(), pci_msi_count(), pci_msix_count(),
  pci_pending_msix(), pci_release_msi(),  pci_remap_msix(), and
  pci_set_max_read_req().
- Group the functions into five sub-sections: raw configuration access,
  locating devices, device information, device configuration, and
  message signaled interrupts.
- Discourage use of pci_disable_io() and pci_enable_io() in device drivers.
  The PCI bus driver handles this automatically as resources are activated.

git-svn-id: svn://svn.freebsd.org/base/stable/9@233395 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f