Update 'vos shadow' man page

Update 'vos shadow' man page

Change the 'vos shadow' man page to say that updating the
VLDB with shadow volumes does only work if the VLDB entries
for the corresponding source volumes are deleted first.

Change-Id: I2764776b7a03346b5b2809f796d1deed0c32933b
Reviewed-on: http://gerrit.openafs.org/8652
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Dan van der Ster <daniel.vanderster@cern.ch>
Tested-by: Dan van der Ster <daniel.vanderster@cern.ch>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>