cxl: doc/linux/access-coordinates Update access coordinates calculation methods

Add documentation on how to calculate the access coordinates for a given
CXL region in detail.

Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/20250515000923.2590820-4-dave.jiang@intel.com


Signed-off-by: Dave Jiang <dave.jiang@intel.com>