PCI: Document the Capability accessor RMW improvements

Documentation claims port service drivers should play nice with respect to
PCIe Capability changes, but the concurrency control is now provided in the
Capability accessors as long as the correct ones are used.

Update the documention to match the RMW accessor behavior.

Link: https://lore.kernel.org/r/20230717120503.15276-12-ilpo.jarvinen@linux.intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>