wipefs: add comments to code
libblkid: no more probe for btrfs backup superblock

 * Linux kernel cares about the first superblock only

 * backup superblock are FS specific stuff and there is no reason to
   care about it in generic tools

 * the problem with broken btrfs utils has been already fixed (it was
   possible to use the utils on filesystem with erased primary
   superblok without any warning message).

Signed-off-by: Karel Zak <kzak@redhat.com>