mark structural types as Scala 3 only

_overviews/scala3-book/types-structural.md

@@ -6,12 +6,18 @@ languages: [ru, zh-cn]
 num: 56
 previous-page: types-opaque-types
 next-page: types-dependent-function
+scala3: true
+versionSpecific: true
 ---
 
 {% comment %}
 NOTE: It would be nice to simplify this more.
 {% endcomment %}
 
+_Scala 2 has a weaker form of structural types based on Java reflection, achieved with `import scala.language.reflectiveCalls`_.
+
+## Introduction
+
 Some use cases, such as modeling database access, are more awkward in statically typed languages than in dynamically typed languages.
 With dynamically typed languages, it’s natural to model a row as a record or object, and to select entries with simple dot notation, e.g. `row.columnName`.