doc: child_process.execSync .stdio default is pipe

PR-URL: #9701
Fixes: #9636
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/child_process.md

@@ -626,7 +626,7 @@ added: v0.11.12
   * `cwd` {String} Current working directory of the child process
   * `input` {String|Buffer} The value which will be passed as stdin to the spawned process
     - supplying this value will override `stdio[0]`
-  * `stdio` {String|Array} Child's stdio configuration.
+  * `stdio` {String|Array} Child's stdio configuration. (Default: 'pipe')
   * `env` {Object} Environment key-value pairs
   * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
   * `gid` {Number} Sets the group identity of the process. (See setgid(2).)