cxl/pci: Disambiguate cxl_pci further from cxl_mem

cxl/pci: Disambiguate cxl_pci further from cxl_mem

Commit 21e9f76733a8 ("cxl: Rename mem to pci") introduced the cxl_pci
driver which had formerly been named cxl_mem. At the time, the goal was
to be as light touch as possible because there were other patches in
flight. Since things have settled now, and a new cxl_mem driver will be
introduced shortly, spend the LOC now to clean up the existing names.

While here, fix the kernel docs to explain the situation better after
the core rework that has already landed.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
Link: https://lore.kernel.org/r/20210913163324.1008564-4-ben.widawsky@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>