doc: child_process: document uid and gid spawn() options

doc/api/child_process.markdown

@@ -250,6 +250,8 @@ there is no IPC channel keeping it alive. When calling this method the
     for stdio.  (See below)
   * `env` {Object} Environment key-value pairs
   * `detached` {Boolean} The child will be a process group leader.  (See below)
+  * `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 object}
 
 Launches a new process with the given `command`, with  command line arguments in `args`.