Documentation: PCI: Sync error recovery doc with code

Amend the documentation on PCI error recovery to fix minor inaccuracies
vis-à-vis the actual code:

* The documentation claims that a missing ->resume() or ->mmio_enabled()
  callback always leads to recovery through reset.  But none of the
  implementations do this (pcie_do_recovery(), eeh_handle_normal_event(),
  zpci_event_do_error_state_clear()).

  Drop the claim to align the documentation with the code.

* The documentation does not list PCI_ERS_RESULT_RECOVERED as a valid
  return value from ->error_detected().  But none of the implementations
  forbid this and some drivers are returning it, e.g.:
  drivers/bus/mhi/host/pci_generic.c
  drivers/infiniband/hw/hfi1/pcie.c

  Further down in the documentation it is implied that the return value is
  in fact allowed:
  "The platform will call the resume() callback on all affected device
  drivers if all drivers on the segment have returned
  PCI_ERS_RESULT_RECOVERED from one of the 3 previous callbacks."

  The "3 previous callbacks" being ->error_detected(), ->mmio_enabled()
  and ->slot_reset().

  Add it to the valid return values for consistency.

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/ed3c3385499775fcc25f1ee66f395e212919f94a.1757942121.git.lukas@wunner.de

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