Documentation: PCI: Amend error recovery doc with DPC/AER specifics

Documentation: PCI: Amend error recovery doc with DPC/AER specifics

Amend the documentation on PCI error recovery with specifics about
Downstream Port Containment and Advanced Error Reporting:

* Explain that with DPC, devices are inaccessible upon an error (similar
  to EEH on powerpc) and do not become accessible until the link is
  re-enabled.

* Explain that with AER, although devices may already be accessible in the
  ->error_detected() callback, accesses should be deferred to the
  ->mmio_enabled() callback for compatibility with EEH on powerpc and with
  s390.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Link: https://patch.msgid.link/61d8eeadb20ee71c3a852f44c863bfe0209c454d.1757942121.git.lukas@wunner.de