Docs: Add a file header to wp-includes/widgets/class-wp-widget-calendar.php, introduced in [33746].

Also clarifies the class DocBlock summary and tags for `WP_Widget_Calendar`.

See #33413. See #33701.


git-svn-id: https://develop.svn.wordpress.org/trunk@34430 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/widgets/class-wp-widget-calendar.php

@@ -1,10 +1,18 @@
 <?php
 /**
- * Calendar widget class
+ * Widget API: WP_Widget_Calendar class
  *
- * @since 2.8.0
  * @package WordPress
  * @subpackage Widgets
+ * @since 4.4.0
+ */
+
+/**
+ * Core class used to implement the Calendar widget.
+ *
+ * @since 2.8.0
+ *
+ * @see WP_Widget
  */
 class WP_Widget_Calendar extends WP_Widget {
 	/**