Docs: Add a missing `@since` version to the DocBlock for `wp_add_dashboard_widget()`.

See #32246.


git-svn-id: https://develop.svn.wordpress.org/trunk@36878 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/dashboard.php

@@ -132,6 +132,8 @@ function wp_dashboard_setup() {
 /**
  * Adds a new dashboard widget.
  *
+ * @since 2.7.0
+ *
  * @global array $wp_dashboard_control_callbacks
  *
  * @param string   $widget_id        Widget ID  (used in the 'id' attribute for the widget).