dm-verity: Document restart_on_error and panic_on_error options

This patch adds documentation for options introduced in commit
f811b838 ("dm-verity: introduce the options restart_on_error and panic_on_error").

Signed-off-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>