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.NonCompanionObject::class.isData)
-    assertFalse(S.NonCompanionObject::class.isInner)
-    assertFalse(S.NonCompanionObject::class.isCompanion)
-    assertFalse(S.NonCompanionObject::class.isFun)
-    assertFalse(S.NonCompanionObject::class.isValue)
+    assertFalse(S.RegularObject::class.isSealed)
+    assertFalse(S.RegularObject::class.isData)
+    assertFalse(S.RegularObject::class.isInner)
+    assertFalse(S.RegularObject::class.isCompanion)
+    assertFalse(S.RegularObject::class.isFun)
+    assertFalse(S.RegularObject::class.isValue)
 
     assertFalse(S.FunInterface::class.isSealed)
     assertFalse(S.FunInterface::class.isData)