^KT-58985 Fixed
Review: https://jetbrains.team/p/kt/reviews/10385
`isData` returns `true` for `data object`s even if kotlin-reflect
version is old (1.2.0), so it looks like we got correct `KClass.isData`
behaviour for "free". That's why this behaviour was never covered with
tests.
This commit updates the documentation and covers the behaviour described
in the KDoc with tests.