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/plugin.php

@@ -2463,6 +2463,8 @@ function resume_plugin( $plugin, $redirect = '' ) {
  * Renders an admin notice in case some plugins have been paused due to errors.
  *
  * @since 5.2.0
+ *
+ * @global string $pagenow
  */
 function paused_plugins_notice() {
 	if ( 'plugins.php' === $GLOBALS['pagenow'] ) {