Docs: Document the usage of $pagenow global in a few functions.

See #51800.

git-svn-id: https://develop.svn.wordpress.org/trunk@50407 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/ms.php

@@ -1018,6 +1018,8 @@ jQuery(document).ready( function($) {
  *
  * @since 4.6.0
  *
+ * @global string $pagenow
+ *
  * @param array $args {
  *     Optional. Array or string of Query parameters. Default empty array.
  *