PCI: rework Documentation/pci.txt

Rewrite Documentation/pci.txt:
o restructure document to match how API is used when writing init code.
o update to reflect changes in struct pci_driver function pointers.
o removed language on "new style vs old style" device discovery.
  "Old style" is now deprecated. Don't use it. Left description in
  to document existing driver behaviors.
o add section "Legacy I/O Port free driver" by Kenji Kaneshige
  http://lkml.org/lkml/2006/11/22/25
  (renamed to "pci_enable_device_bars() and Legacy I/O Port space")
o add "MMIO space and write posting" section to help avoid common pitfall
  when converting drivers from IO Port space to MMIO space.
  Orignally posted http://lkml.org/lkml/2006/2/27/24
o many typo/grammer/spelling corrections from Randy Dunlap
o two more spelling corrections from Stephan Richter
o fix CodingStyle as per Randy Dunlap
Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>