mark value classes as scala 2 only

Author: bishabosha

_overviews/core/value-classes.md

@@ -7,9 +7,11 @@ partof: value-classes
 languages: [ja, zh-cn]
 
 permalink: /overviews/core/:title.html
+scala2: true
+versionSpecific: true
 ---
 
-**Mark Harrah**
+In Scala 3, a similar result can be achieved with [opaque types][opaques].
 
 ## Introduction
 
@@ -212,7 +214,7 @@ Note that local classes, traits, and objects are not allowed either, as in the f
         ...
       }
     }
-    
+
     Local.scala:3: error: implementation restriction: nested class is not allowed in value class
       class Local
             ^
@@ -272,3 +274,5 @@ but this is allowed because the enclosing object is top-level:
     object Outer {
       class Inner(val x: Int) extends AnyVal
     }
+
+[opaques]: {% link _overviews/scala3-book/types-opaque-types.md %}

_overviews/scala3-book/types-opaque-types.md

@@ -6,9 +6,12 @@ languages: [ru, zh-cn]
 num: 55
 previous-page: types-variance
 next-page: types-structural
+scala3: true
+versionSpecific: true
 ---
 
-Scala 3 _Opaque type aliases_ provide type abstractions without any **overhead**.
+_Opaque type aliases_ provide type abstraction without any **overhead**.
+In Scala 2, a similar result could be achieved with [value classes][value-classes].
 
 ## Abstraction Overhead
 
@@ -142,3 +145,4 @@ As illustrated above, opaque types are convenient to use, and integrate very wel
 
 
 [extension]: {% link _overviews/scala3-book/ca-extension-methods.md %}
+[value-classes]: {% link _overviews/core/value-classes.md %}