cxl/uapi: Add warning on CXL command enum

cxl/uapi: Add warning on CXL command enum

The CXL command enum is exported to user space and must maintain
backwards compatibility.

Add comment that new defines must be added to the end of the list.

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20221222-cxl-misc-v4-2-62f701c1cdd1@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>