doc: child_process .stdio accepts a String type

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

PR-URL: #9637
Fixes: #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>

doc/api/child_process.md

@@ -575,7 +575,7 @@ added: v0.11.12
   * `input` {String|Buffer} The value which will be passed as stdin to the
     spawned process
     - supplying this value will override `stdio[0]`
-  * `stdio` {Array} Child's stdio configuration. (Default: `'pipe'`)
+  * `stdio` {String | Array} Child's stdio configuration. (Default: `'pipe'`)
     - `stderr` by default will be output to the parent process' stderr unless
       `stdio` is specified
   * `env` {Object} Environment key-value pairs
@@ -613,7 +613,7 @@ added: v0.11.12
   * `input` {String|Buffer} The value which will be passed as stdin to the
     spawned process
     - supplying this value will override `stdio[0]`
-  * `stdio` {Array} Child's stdio configuration. (Default: `'pipe'`)
+  * `stdio` {String | Array} Child's stdio configuration. (Default: `'pipe'`)
     - `stderr` by default will be output to the parent process' stderr unless
       `stdio` is specified
   * `env` {Object} Environment key-value pairs
@@ -657,7 +657,7 @@ added: v0.11.12
   * `input` {String|Buffer} The value which will be passed as stdin to the
     spawned process
     - supplying this value will override `stdio[0]`
-  * `stdio` {Array} Child's stdio configuration.
+  * `stdio` {String | Array} Child's stdio configuration.
   * `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).)