Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 27558)
+++ src/wp-includes/theme.php	(working copy)
@@ -1379,7 +1379,8 @@
  * The init hook may be too late for some features.
  *
  * @since 2.9.0
- * @param string $feature the feature being added
+ * @param string $feature The feature being added.
+ * @return mixed False if $feature is 'html5' and not passing an array as 2nd argument (empty 2nd arg is deprecated usage).
  */
 function add_theme_support( $feature ) {
 	global $_wp_theme_features;
@@ -1398,6 +1399,7 @@
 		case 'html5' :
 			// You can't just pass 'html5', you need to pass an array of types.
 			if ( empty( $args[0] ) ) {
+				// This is deprecated usage, only here for back compat
 				$args = array( 0 => array( 'comment-list', 'comment-form', 'search-form' ) );
 			} elseif ( ! is_array( $args[0] ) ) {
 				_doing_it_wrong( "add_theme_support( 'html5' )", 'You need to pass an array of types.', '3.6.1' );