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>

Documentation PCI: Fix pciebus-howto.rst filename typo

2e6422444894 ("Documentation: PCI: convert PCIEBUS-HOWTO.txt to reST")
incorrectly renamed PCIEBUS-HOWTO.txt to picebus-howto.rst.

Rename it to pciebus-howto.rst.

Fixes: 2e6422444894 ("Documentation: PCI: convert PCIEBUS-HOWTO.txt to reST")
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>