libnvdimm, pmem: clarify the write+clear_poison+write flow

The ACPI specification does not specify the state of data after a clear
poison operation.  Potential future libnvdimm bus implementations for
other architectures also might not specify or disagree on the state of
data after clear poison.  Clarify why we write twice.

Reported-by: Jeff Moyer <jmoyer@redhat.com>
Reported-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>