doc: add Windows section to fs newcell

doc: add Windows section to fs newcell

fs newcell works slightly differently on Windows. Document the
differences in the man page.

FIXES 125094

Change-Id: I3fb71ebb28290222069d795762f50a82f3fb6495
Reviewed-on: http://gerrit.openafs.org/6919
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementix.org>