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

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>
(cherry picked from commit 186158c9eb2379ec350831b8fb244c51ae234a6e)

Change-Id: Ie0daa8631c45eb457c5eb7a1e80a1f70401127d6
Reviewed-on: http://gerrit.openafs.org/7493
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>