EDAC: Update documentation for the CXL memory patrol scrub control feature

EDAC: Update documentation for the CXL memory patrol scrub control feature

Update the Documentation/edac/scrub.rst to include use cases and
policies for CXL memory device-based, CXL region-based patrol scrub
control and CXL Error Check Scrub (ECS).

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Shiju Jose <shiju.jose@huawei.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20250521124749.817-2-shiju.jose@huawei.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>