RFR (XS) 8148730: Add @since tags in new String concat APIs

[Aleksey Shipilev]

Hi,

Please review this tiny update, that puts @since 9 tags over new Indify
String Concat JDK APIs:
  http://cr.openjdk.java.net/~shade/8148730/webrev.01/

Cheers,
-Aleksey