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-includes/load.php

@@ -938,6 +938,8 @@ function wp_is_recovery_mode() {
  *
  * @since 5.2.0
  *
+ * @global string $pagenow
+ *
  * @return bool True if the current endpoint should be protected.
  */
 function is_protected_endpoint() {