doc: 'ipc' is required with fork stdio option

Fixes: nodejs#8167
PR-URL: nodejs#8290
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

doc/api/child_process.md

@@ -263,6 +263,8 @@ added: v0.5.0
     [`stdio`][] for more details (Default: `false`)
   * `stdio` {Array} Supports the array version of [`child_process.spawn()`][]'s
     [`stdio`][] option. When this option is provided, it overrides `silent`.
+    The array must contain exactly one item with value `'ipc'` or an error will
+    be thrown. For instance `[0, 1, 2, 'ipc']`.
   * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
   * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
 * Return: {ChildProcess}