Windows: Comment CommonWrite VDL Change

Add a comment reminding the reader that CcSetFileSizes only needs
to be called on a ValidDataLength change if the VDL value has decreased.
A write operation cannot result in a decrease therefore CcSetFileSizes
does not need to be called from within AFSCommonWrite().

Change-Id: Iaf867ec876a6265dc2c8a7ba2319fdf67503a467
Reviewed-on: http://gerrit.openafs.org/9757
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>