Docs: Add an entry for `$pagenow` global in `WP_Customize_Manager::setup_theme()`.

Props mt8.biz.
Fixes #40297.

git-svn-id: https://develop.svn.wordpress.org/trunk@40349 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-customize-manager.php

@@ -462,6 +462,8 @@ final class WP_Customize_Manager {
 	 * Check if customize query variable exist. Init filters to filter the current theme.
 	 *
 	 * @since 3.4.0
+	 *
+	 * @global string $pagenow
 	 */
 	public function setup_theme() {
 		global $pagenow;