doc: fixed child_process.exec doc

Changed child_process.exec documentation to reflect that the parameters stdout and stderr are instances of Buffer only if {encoding: null}. Otherwise, they are instances of String with the encoding.

Fixes #9434

removed .DS_Store

PR-URL: https://github.com/joyent/node/pull/14088


Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>
Reviewed-By: piscisaureus - Bert Belder <bertbelder@gmail.com>