cxl: Sync up the driver-api/cxl documentation

cxl: Sync up the driver-api/cxl documentation

pmem.c regs.c mbox.c identifiers were missing. Add them to
memory-devices.rst following their respective DOC comment includes.

Two acpi.c identifiers were available, but not used in kernel-doc's:
1) Add add_cxl_resources to memory-devices.rst and fix up the Sphinx
complaint on the ascii art by escaping it.
2) Add cxl_acpi_evaluate_qtg_dsm to access-coordinates.rst.

core/features.c is new. Add a "DOC: cxl features" comment to the
source and identifiers to memory_devices.rst.

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://patch.msgid.link/20250513215813.1419645-1-alison.schofield@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>