doc: include the optional options parameter

`mkdtemp` functions accept an optional `options` parameter, which can
be either a String specifying encoding, or an Object with an `encoding`
property.

PR-URL: https://github.com/nodejs/node/pull/7842


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>