Correct the return value doc block for get_theme_support().

See #30224.


git-svn-id: https://develop.svn.wordpress.org/trunk@30188 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.php

@@ -1668,7 +1668,7 @@ add_action( 'wp_loaded', '_custom_header_background_just_in_time' );
  * @since 3.1.0
  *
  * @param string $feature the feature to check
- * @return array The array of extra arguments
+ * @return mixed The array of extra arguments or value of registered featured.
  */
 function get_theme_support( $feature ) {
 	global $_wp_theme_features;