Update Documentation/driver-api/pci/p2pdma.rst doc and remove references to
obsolete p2pdma mapping functions.

Fixes: 0d06132f ("PCI/P2PDMA: Remove pci_p2pdma_[un]map_sg()")
Link: https://lore.kernel.org/r/20231113180325.444692-1-tstruk@gmail.comSigned-off-by: Tadeusz Struk <tstruk@gigaio.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Cc: stable@kernel.org