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.

compiler/testData/codegen/box/reflection/modifiers/classes.kt

@@ -7,9 +7,10 @@ import kotlin.test.assertFalse
 
 sealed class S {
     data class DataClass(val x: Int) : S()
+    data object DataObject
     inner class InnerClass
     companion object
-    object NonCompanionObject
+    object RegularObject
     fun interface FunInterface { fun invoke() }
 }
 
@@ -34,6 +35,13 @@ fun box(): String {
     assertFalse(S.DataClass::class.isFun)
     assertFalse(S.DataClass::class.isValue)
 
+    assertFalse(S.DataObject::class.isSealed)
+    assertTrue(S.DataObject::class.isData)
+    assertFalse(S.DataObject::class.isInner)
+    assertFalse(S.DataObject::class.isCompanion)
+    assertFalse(S.DataObject::class.isFun)
+    assertFalse(S.DataObject::class.isValue)
+
     assertFalse(S.InnerClass::class.isSealed)
     assertFalse(S.InnerClass::class.isData)
     assertTrue(S.InnerClass::class.isInner)
@@ -48,12 +56,12 @@ fun box(): String {
     assertFalse(S.Companion::class.isFun)
     assertFalse(S.Companion::class.isValue)
 
-    assertFalse(S.NonCompanionObject::class.isSealed)
+    assertFalse(S.RegularObject::class.isSealed)
-    assertFalse(S.NonCompanionObject::class.isData)
+    assertFalse(S.RegularObject::class.isData)
-    assertFalse(S.NonCompanionObject::class.isInner)
+    assertFalse(S.RegularObject::class.isInner)
-    assertFalse(S.NonCompanionObject::class.isCompanion)
+    assertFalse(S.RegularObject::class.isCompanion)
-    assertFalse(S.NonCompanionObject::class.isFun)
+    assertFalse(S.RegularObject::class.isFun)
-    assertFalse(S.NonCompanionObject::class.isValue)
+    assertFalse(S.RegularObject::class.isValue)
 
     assertFalse(S.FunInterface::class.isSealed)
     assertFalse(S.FunInterface::class.isData)

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.
+     * `true` if this class or object has the `data` keyword.
-     * See the [Kotlin language documentation](https://kotlinlang.org/docs/reference/data-classes.html)
+     * For more information, see [data class](https://kotlinlang.org/docs/reference/data-classes.html) and
-     * for more information.
+     * [data object](https://kotlinlang.org/docs/object-declarations.html#data-objects)
+     * in the Kotlin language documentation.
      */
     @SinceKotlin("1.1")
     public val isData: Boolean