doc: volinfo manpage fixes

We now have a -checkout option, but no -online option. Document
-checkout and remove -online.

Change-Id: Ie8d6e35c6e09abf994b8417b3bb20ee379095a25
Reviewed-on: http://gerrit.openafs.org/10407
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>