Docs: Improve rejects() and extend() examples, upgrade guide fix

* Fix bug in upgrade guide where I mistakenly started using arrow
  functions in an example that involves text context. Happened in
  commit qunitjs/qunitjs.com@6ab0198592.

* Improve rejects() example by simplifying the code and spelling out
  what would happen in a way that's easy to try out and experience
  yourself.

* Clarify wording in extend() example.

Author: Krinkle

docs/api/assert/async.md

@@ -75,10 +75,10 @@ The `count` parameter can be used to require multiple calls to the same callback
 
 ```js
 function uploadBatch (batch, notify, complete) {
-  batch.forEach((item) => {
+  for (const item of batch) {
     // Do something with item
     notify();
-  });
+  }
   complete(null);
 }
 

docs/api/assert/rejects.md

@@ -94,28 +94,23 @@ QUnit.test('rejects example', assert => {
 The `assert.rejects()` method returns a `Promise` which handles the (often asynchronous) resolution and rejection logic for test successes and failures. It is not required to `await` the returned value, since QUnit internally handles the async control for you and waits for a settled state. However, if your test code requires a consistent and more isolated state between `rejects` calls, then this should be explicitly awaited to hold back the next statements.
 
 ```js
-QUnit.test('stateful rejects example', async assert => {
+QUnit.test('stateful example', async function (assert) {
   let value;
 
-  // asynchronously resolve if value < 5, and reject otherwise
-  function asyncChecker () {
-    return new Promise((resolve, reject) => {
-      setTimeout(() => {
-        if (value < 5) {
-          resolve();
-        } else {
-          reject('bad value: ' + value);
-        }
-      }, 10);
-    });
+  async function feedMe () {
+    await Promise.resolve();
+    if (value === 1) {
+      throw new Error('I want more');
+    }
+    throw new Error('Thank you');
   }
 
-  value = 8;
-  await assert.rejects(asyncChecker(), /bad value: 8/);
+  value = 5;
+  // if we don't await, then this test will fail,
+  // because `value = 1` would run too soon
+  await assert.rejects(feedMe(), /Thank you/);
 
-  // if the above was not awaited, then the next line would change the value
-  // before the previous assertion could occur, and would cause a test failure
-  value = Infinity;
-  await assert.rejects(asyncChecker(), /bad value: Infinity/);
+  value = 1;
+  await assert.rejects(feedMe(), /I want more/);
 });
 ```

docs/api/extension/QUnit.extend.md

@@ -28,21 +28,21 @@ This method will modify the `target` object to contain the "own" properties defi
 Use `QUnit.extend` to merge two objects.
 
 ```js
-QUnit.test('QUnit.extend', assert => {
+QUnit.test('using extend', function (assert) {
   const base = {
     a: 1,
     b: 2,
-    z: 3
+    zed: 3
   };
   QUnit.extend(base, {
     b: 2.5,
     c: 3,
-    z: undefined
+    zed: undefined
   });
 
-  assert.strictEqual(base.a, 1, 'Unspecified values are not modified');
-  assert.strictEqual(base.b, 2.5, 'Existing values are updated');
-  assert.strictEqual(base.c, 3, 'New values are defined');
-  assert.false('z' in base, 'Values specified as `undefined` are removed');
+  assert.strictEqual(base.a, 1, 'Unspecified keys remain unchanged');
+  assert.strictEqual(base.b, 2.5, 'Matching keys are replaced');
+  assert.strictEqual(base.c, 3, 'New keys are added');
+  assert.false('zed' in base, 'Keys with the value `undefined` are removed');
 });
 ```

docs/upgrade-guide-2.x.md

@@ -134,10 +134,10 @@ After:
 
 ```js
 QUnit.module('router', {
-  beforeEach: () => {
+  beforeEach () {
     this.router = new Router();
   },
-  afterEach: () => {
+  afterEach () {
     this.router.destroy();
   }
 });