Update Configuration.md (jestjs#7455)

docs/Configuration.md

@@ -333,6 +333,26 @@ Default: `undefined`
 
 This option allows the use of a custom global setup module which exports an async function that is triggered once before all test suites. This function gets Jest's `globalConfig` object as a parameter.
 
+_Note: Any global variables that are defined through `globalSetup` can only be read in `globalTeardown`. You cannot retrieve globals defined here in your test suites._
+
+Example:
+
+```js
+// setup.js
+module.exports = async () => {
+  // ...
+  // Set reference to mongod in order to close the server during teardown.
+  global.__MONGOD__ = mongod;
+};
+```
+
+```js
+// teardown.js
+module.exports = async function() {
+  await global.__MONGOD__.stop();
+};
+```
+
 ### `globalTeardown` [string]
 
 Default: `undefined`