vol: add comment nt_unlink cannot with fopen handles

vol: add comment nt_unlink cannot with fopen handles

On Windows, files opened via the CRT fopen or open
POSIX compatibility functions cannot be marked for delete
on close.  Add a comment until such time as the use of
fopen and open on Windows can be replaced.

Reviewed-on: http://gerrit.openafs.org/3707
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
(cherry picked from commit eb6f3a89a3f4da6d3de2d4ae34743014e4eb0fb2)

Change-Id: I6d66617b53f46e832d72ac8ed9a41730fafca521
Reviewed-on: http://gerrit.openafs.org/3854
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>