cxl: Add documentation to explain the shared link bandwidth calculation

Create a kernel documentation to describe how the CXL shared upstream
link bandwidth is calculated.

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20240904001316.1688225-4-dave.jiang@intel.com


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