doc: add `added:` info for `string_decoder`

PR-URL: https://github.com/nodejs/node/pull/6741
Ref: https://github.com/nodejs/node/issues/6578


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>