storage: Document wiping formatted volume types

storage: Document wiping formatted volume types

When wiping a volume we just rewrite all the data of the volume, not
only the content.  Since format gets overridden, we need to recreate the
volume.  However we can't do that for every possible format out there.
Since it was only coded for the ploop volume type, let's document what
might be the consequences instead of forbidding it for every other
format out there.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=868771

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>