child_process: update outdated comment

PR-URL: https://github.com/nodejs/node/pull/8988/
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/child_process.js

@@ -45,8 +45,8 @@ exports.fork = function(modulePath /*, args, options*/) {
   args = execArgv.concat([modulePath], args);
 
   if (!Array.isArray(options.stdio)) {
-    // Leave stdin open for the IPC channel. stdout and stderr should be the
-    // same as the parent's if silent isn't set.
+    // Use a separate fd=3 for the IPC channel. Inherit stdin, stdout,
+    // and stderr from the parent if silent isn't set.
     options.stdio = options.silent ? ['pipe', 'pipe', 'pipe', 'ipc'] :
         [0, 1, 2, 'ipc'];
   } else if (options.stdio.indexOf('ipc') === -1) {