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=868771Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/libvirt-storage.c

@@ -1725,11 +1725,16 @@ virStorageVolDelete(virStorageVolPtr vol,
  * @vol: pointer to storage volume
  * @flags: extra flags; not used yet, so callers should always pass 0
  *
- * Ensure data previously on a volume is not accessible to future
- * reads. Also note, that depending on the actual volume
- * representation, this call may not really overwrite the
- * physical location of the volume. For instance, files stored
- * journaled, log structured, copy-on-write, versioned, and
+ * Ensure data previously on a volume is not accessible to future reads.
+ *
+ * The data to be wiped may include the format and possibly size information,
+ * so non-raw images might become raw with a different size. It is storage
+ * backend dependent whether the format and size information is regenerated
+ * once the initial volume wipe is completed.
+ *
+ * Depending on the actual volume representation, this call may not
+ * overwrite the physical location of the volume. For instance, files
+ * stored journaled, log structured, copy-on-write, versioned, and
  * network file systems are known to be problematic.
  *
  * Returns 0 on success, or -1 on error