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

Follow-up to [53060].

See #54729.
Built from https://develop.svn.wordpress.org/trunk@53061


git-svn-id: http://core.svn.wordpress.org/trunk@52650 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/admin.php

@@ -124,7 +124,7 @@ wp_enqueue_script( 'common' );
  * @global array  $wp_importers
  * @global string $hook_suffix
  * @global string $plugin_page
- * @global string $typenow
+ * @global string $typenow      The post type of the current screen.
  * @global string $taxnow
  */
 global $pagenow, $wp_importers, $hook_suffix, $plugin_page, $typenow, $taxnow;