doc: clarify stdout/stderr arguments to callback

Clarify that the arguments to child_process.execFile
and child_process.exec callback can be Buffer or strings.

Fixes: https://github.com/nodejs/node/issues/3389
PR-URL: https://github.com/nodejs/node/pull/6015


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>