Document field values of instance constructed via `NewTombstone`.
Assert the values in corresponding unit test.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
Document that `NewTombstoneFromV2` function return
`nil` when called on `nil`. Write corresponding
unit tests.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>