Docs: Add missing description for $pagenow global in various functions.

See #54729, #55499.

git-svn-id: https://develop.svn.wordpress.org/trunk@53060 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/admin-bar.php

@@ -1233,7 +1233,7 @@ function show_admin_bar( $show ) {
  * @since 3.1.0
  *
  * @global bool   $show_admin_bar
- * @global string $pagenow
+ * @global string $pagenow        The filename of the current screen.
  *
  * @return bool Whether the admin bar should be showing.
  */