Docs: Add a `@global` entry for `$wpdb` in `_get_cron_lock`.

Props tejas5989.
Fixes #41433.

git-svn-id: https://develop.svn.wordpress.org/trunk@41146 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-cron.php

@@ -34,6 +34,8 @@ if ( !defined('ABSPATH') ) {
  * @ignore
  * @since 3.3.0
  *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
  * @return string|false Value of the `doing_cron` transient, 0|false otherwise.
  */
 function _get_cron_lock() {