doc: add `added:` information for assert

PR-URL: https://github.com/nodejs/node/pull/6688
Ref: https://github.com/nodejs/node/issues/6578


Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>