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.

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>
(cherry picked from commit 83ccb62012d7b3b696dd48e05feb8adeb203463a)

Change-Id: I59eacb7406e8bec9e5736afe32564a04be7ff72a
Reviewed-on: http://gerrit.openafs.org/8786
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>