Document field values of instance constructed via `NewPlacementPolicy`.
Assert the values in corresponding unit test.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
Document that `PlacementPolicy.ToV2` method return `nil`
when called on `nil`. Document that `PlacementPolicyFromV2`
function return `nil` when called on `nil`. Write corresponding
unit tests.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
