x86/PCI: Clarify AMD Fam10h config access restrictions comment

Clarify the comment about AMD Fam10h config access restrictions, fix typos,
and add a reference to the specification.

[bhelgaas: streamline]
Signed-off-by: Tomasz Nowicki <tomasz.nowicki@linaro.org>
Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>