Build/Test tools: Document the return value of `WP_UnitTestCase::factory()`.

See #41017


git-svn-id: https://develop.svn.wordpress.org/trunk@41945 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/testcase.php

@@ -33,6 +33,11 @@ class WP_UnitTestCase extends PHPUnit_Framework_TestCase {
  	    }
  	}
 
+	/**
+	 * Fetches the factory object for generating WordPress fixtures.
+	 *
+	 * @return WP_UnitTest_Factory The fixture factory.
+	 */
 	protected static function factory() {
 		static $factory = null;
 		if ( ! $factory ) {