virStorageVolWipe: Document that wiping journaled FS is useless

virStorageVolWipe: Document that wiping journaled FS is useless

So you have a libvirt volume that you want to wipe out. But lets
say that the volume is actually a file stored on a journaled
filesystem. Overwriting it with zeroes or a pattern does not mean
that corresponding physical location on the disk is overwritten
too, due to journaling. It's the same story with network based
volumes, copy-on-write filesystems, and so on. Since there is no
way that an userland application can write onto specific areas on
disk, all that we can do is document the fact.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>