Update KClass.isData KDoc to include info about data objects

^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.

libraries/stdlib/jvm/src/kotlin/reflect/KClass.kt

@@ -104,9 +104,10 @@ public actual interface KClass<T : Any> : KDeclarationContainer, KAnnotatedEleme
     public val isSealed: Boolean
 
     /**
-     * `true` if this class is a data class.
-     * See the [Kotlin language documentation](https://kotlinlang.org/docs/reference/data-classes.html)
-     * for more information.
+     * `true` if this class or object has the `data` keyword.
+     * For more information, see [data class](https://kotlinlang.org/docs/reference/data-classes.html) and
+     * [data object](https://kotlinlang.org/docs/object-declarations.html#data-objects)
+     * in the Kotlin language documentation.
      */
     @SinceKotlin("1.1")
     public val isData: Boolean