8148730: Add @since tags in new String concat APIs
Reviewed-by: alanb

8148483: JEP 280: Indify String Concatenation
Reviewed-by: psandoz, mcimadamore, igerasim, forax, plevart, vlivanov, ihse
Contributed-by: Aleksey Shipilev <aleksey.shipilev@oracle.com>, Remi Forax <forax@univ-mlv.fr>, Peter Levart <peter.levart@gmail.com>