Docs: Update the return notation for `get_current_screen()` to note that `null` can also be returned if the screen has not been defined.

Props screamingdev.
Fixes #36382.

Built from https://develop.svn.wordpress.org/trunk@37534


git-svn-id: http://core.svn.wordpress.org/trunk@37502 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/screen.php

@@ -200,7 +200,7 @@ function add_screen_option( $option, $args = array() ) {
  *
  * @global WP_Screen $current_screen
  *
- * @return WP_Screen Current screen object
+ * @return WP_Screen|null Current screen object or null when screen not defined.
  */
 function get_current_screen() {
 	global $current_screen;

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.6-alpha-37533';
+$wp_version = '4.6-alpha-37534';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.