doc: update information about vlserver logging

doc: update information about vlserver logging

Mention the vlserver -d option can be used to set the initial logging
level.

Thanks to Mark Vitale for the suggestion.

Reviewed-on: https://gerrit.openafs.org/12324
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit f5f057ce8198480fb9c67f2a8c8eee906f8a7c4a)

Change-Id: Iaa0f10d020d3993fe92690c860cdad03605d31ec
Reviewed-on: https://gerrit.openafs.org/12477
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>