doc: document volinfo -filenames option

Add the namei -filenames option to the volinfo man page.

Note this option as implemented implies the -vnode flag, but for
consistency with the other fields that modify the default mode
output, it is documented to be accompany the -vnode flag.

Reviewed-on: http://gerrit.openafs.org/4690
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit e3fb2740ba20e78fdc93308b6a2843454a6d269c)

Change-Id: I1ef65090d5449f600ef03171436a27c142e69fbb
Reviewed-on: http://gerrit.openafs.org/9563
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>