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>

doc/api/child_process.markdown

@@ -544,8 +544,10 @@ See also: `child_process.exec()` and `child_process.fork()`
   * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
 * `callback` {Function} called with the output when process terminates
   * `error` {Error}
-  * `stdout` {Buffer}
+  * `stdout` {Buffer} only if {encoding: null}, otherwise {String}
-  * `stderr` {Buffer}
+  encoded with the specified encoding or {String} (Default: 'utf8').
+  * `stderr` {Buffer} only if {encoding: null}, otherwise {String}
+  encoded with the specified encoding or {String} (Default: 'utf8').
 * Return: ChildProcess object
 
 Runs a command in a shell and buffers the output.