doc: child_process .stdio accepts a String type

Document that `execFileSync`, `execSync` and `spawnSync` also support
`stdio` as an Array.

PR-URL: https://github.com/nodejs/node/pull/9637
Fixes: https://github.com/nodejs/node/issues/9636


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sam Roberts <sam@strongloop.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>