Add samples for joinTo

KT-20357

libraries/tools/kotlin-stdlib-gen/src/templates/Strings.kt

@@ -30,6 +30,8 @@ object StringJoinOps : TemplateGroupBase() {
 
             If the collection could be huge, you can specify a non-negative value of [limit], in which case only the first [limit]
             elements will be appended, followed by the [truncated] string (which defaults to "...").
+            
+            @sample samples.collections.Collections.Transformations.joinTo
             """
         }
         sequenceClassification(terminal)