doc: document volinfo -filenames option

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.

Change-Id: I7cc5cdf5e74e65ca25ecc086e550a468b803b971
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>