cxl/Documentation: Add more description about min/max scrub cycle

cxl/Documentation: Add more description about min/max scrub cycle

user can configurare scrub cycle for a region or a memory device via
sysfs interface. Currently, these interfaces have not enough description
for the return value. So adding return value description to these
interfaces.

Suggested-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Shiju Jose <shiju.jose@huawei.com>
Signed-off-by: Li Ming <ming.li@zohomail.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Link: https://patch.msgid.link/20250603104314.25569-2-ming.li@zohomail.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>