Docs: Correct @return value for update_core() and Core_Upgrader::upgrade().

See #48303.

git-svn-id: https://develop.svn.wordpress.org/trunk@47099 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-core-upgrader.php

@@ -57,7 +57,7 @@ class Core_Upgrader extends WP_Upgrader {
 	 *        @type bool $do_rollback      Whether to perform this "upgrade" as a rollback.
 	 *                                     Default false.
 	 * }
-	 * @return null|false|WP_Error Null on success, false or WP_Error on failure.
+	 * @return string|false|WP_Error New WordPress version on success, false or WP_Error on failure.
 	 */
 	public function upgrade( $current, $args = array() ) {
 		global $wp_filesystem;