Docs: Expand file header for WP_Importer class.

Follow-up to [14760].

Props huzaifaalmesbah.
See #64224.

git-svn-id: https://develop.svn.wordpress.org/trunk@61523 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/class-wp-importer.php

@@ -1,7 +1,12 @@
 <?php
 /**
  * WP_Importer base class
+ *
+ * @package WordPress
+ * @subpackage Importer
+ * @since 3.0.0
  */
+
 #[AllowDynamicProperties]
 class WP_Importer {
 	/**