Documentation: PCI: Tidy AER documentation

Consistently use:

  PCIe          previously PCIe, PCI Express, or pci express
  Root Port     previously Root Port or root port
  Endpoint      previously EndPoint or endpoint
  AER           previously AER or aer
  please        previously pls

Also update a few awkward wordings.

Link: https://lore.kernel.org/r/20230609222500.1267795-5-helgaas@kernel.orgSigned-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>