Docs: Document the global used in upgrade_590() function.

Follow-up to [51917].

See #53399.


git-svn-id: https://develop.svn.wordpress.org/trunk@52360 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/upgrade.php

@@ -2261,6 +2261,8 @@ function upgrade_560() {
  *
  * @ignore
  * @since 5.9.0
+ *
+ * @global int $wp_current_db_version The old (current) database version.
  */
 function upgrade_590() {
 	global $wp_current_db_version;