Documentation: Explain pci=conf1,conf2 more verbosely

People complained that setting the PCI config space access mechanism
through "pci=conf1" or "pci=conf2" on the command line is not really
documented. Yeah, can you blame them? Look at what we have now.

So try to improve the situation a bit by explaining what those "conf1"
and "conf2" things actually mean.

See http://wiki.osdev.org/PCI

 for more info.

Suggested-by: Eric Morton <Eric.Morton@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
[jc: Added the above URL to the document too]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>