doc: add `added:` info for `process.cpuUsage`

Refs: https://github.com/nodejs/node/issues/6578
PR-URL: https://github.com/nodejs/node/pull/6863


Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>