dm: verity fix documentation

Veritysetup is now part of cryptsetup package.
Remove on-disk header description (which is not parsed in kernel)
and point users to cryptsetup where it the format is documented.
Mention units for block size paramaters.
Fix target line specification and dmsetup parameters.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Alasdair G Kergon <agk@redhat.com>