doc: volscan man page

Provide a man page for the volscan utility

Reviewed-on: http://gerrit.openafs.org/10905
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit a30b98c97d6fbf87018bcb6943e09c1c75a3918d)

Change-Id: I980dd7842bd1c606cb09ee838a584b83d004ccb1
Reviewed-on: http://gerrit.openafs.org/11280
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>