ACPI/nfit: Adjust annotation for why return 0 if fail to find NFIT at start

Add detailed explanation for why it's ok to return 0 if we fail to find
an NFIT at startup. Refer to chapter 9.20.2 NVDIMM Root Device in ACPI
6.2 spec.

Signed-off-by: Ocean He <hehy1@lenovo.com>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>