doc: clarify the statement in vm.createContext()

PR-URL: https://github.com/nodejs/node/pull/10519
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

doc/api/vm.md

@@ -212,8 +212,24 @@ that sandbox][contextified] so that it can be used in calls to
 [`vm.runInContext()`][] or [`script.runInContext()`][]. Inside such scripts,
 the `sandbox` object will be the global object, retaining all of its existing
 properties but also having the built-in objects and functions any standard
-[global object][] has. Outside of scripts run by the vm module, `sandbox` will
-remain unchanged.
+[global object][] has. Outside of scripts run by the vm module, global variables
+will remain unchanged.
+
+```js
+const util = require('util');
+const vm = require('vm');
+
+var globalVar = 3;
+
+const sandbox = { globalVar: 1 };
+vm.createContext(sandbox);
+
+vm.runInContext('globalVar *= 2;', sandbox);
+
+console.log(util.inspect(sandbox)); // 2
+
+console.log(util.inspect(globalVar)); // 3
+```
 
 If `sandbox` is omitted (or passed explicitly as `undefined`), a new, empty
 [contextified][] sandbox object will be returned.