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.

Change-Id: I9cec46bef332c981b2f64ba85e310783e3c71bc9
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>