minor #29681 [Stopwatch] Fixed phpdoc for category name (samnela)

This PR was merged into the 3.4 branch.

Discussion
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[Stopwatch] Fixed phpdoc for category name

| Q             | A
| ------------- | ---
| Branch?       | 3.4 <!-- see below -->
| Bug fix?      | yes
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | n/a   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | n/a <!-- required for new features -->

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Commits
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7f46dfb [Stopwatch] Fixed phpdoc for category name

src/Symfony/Component/Stopwatch/Stopwatch.php

@@ -96,8 +96,8 @@ public function stopSection($id)
     /**
      * Starts an event.
      *
-     * @param string $name     The event name
-     * @param string $category The event category
+     * @param string      $name     The event name
+     * @param string|null $category The event category
      *
      * @return StopwatchEvent
      */

src/Symfony/Component/Stopwatch/Tests/StopwatchTest.php

@@ -35,6 +35,14 @@ public function testStart()
         $this->assertSame($event, $stopwatch->getEvent('foo'));
     }
 
+    public function testStartWithoutCategory()
+    {
+        $stopwatch = new Stopwatch();
+        $stopwatchEvent = $stopwatch->start('bar');
+        $this->assertSame('default', $stopwatchEvent->getCategory());
+        $this->assertSame($stopwatchEvent, $stopwatch->getEvent('bar'));
+    }
+
     public function testIsStarted()
     {
         $stopwatch = new Stopwatch();