Docs: Add missing description for $current_screen global.

Props mukesh27.
Fixes #45604. See #47110.

git-svn-id: https://develop.svn.wordpress.org/trunk@45740 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/admin-header.php

@@ -16,7 +16,7 @@ if ( ! defined( 'WP_ADMIN' ) ) {
  *
  * @global string    $title
  * @global string    $hook_suffix
- * @global WP_Screen $current_screen
+ * @global WP_Screen $current_screen     WordPress current screen object.
  * @global WP_Locale $wp_locale          WordPress date and time locale object.
  * @global string    $pagenow
  * @global string    $update_title

src/wp-admin/includes/class-wp-screen.php

@@ -392,7 +392,7 @@ final class WP_Screen {
 	 * @see set_current_screen()
 	 * @since 3.3.0
 	 *
-	 * @global WP_Screen $current_screen
+	 * @global WP_Screen $current_screen WordPress current screen object.
 	 * @global string    $taxnow
 	 * @global string    $typenow
 	 */

src/wp-admin/includes/screen.php

@@ -215,7 +215,7 @@ function add_screen_option( $option, $args = array() ) {
  *
  * @since 3.1.0
  *
- * @global WP_Screen $current_screen
+ * @global WP_Screen $current_screen WordPress current screen object.
  *
  * @return WP_Screen|null Current screen object or null when screen not defined.
  */