cxl, doc: Remove isonum.txt inclusion

cxl, doc: Remove isonum.txt inclusion

This patch removes the line to include:: <isonum.txt>. From Jon:

"This include has been cargo-culted around the docs...the only real
use of it is to write |copy| rather than ©, but these docs don't even
do that. It can be taken out."

Cc: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Robert Richter <rrichter@amd.com>
Link: https://patch.msgid.link/20260203173604.1440334-1-rrichter@amd.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>