Issue #2501403 by Cottser, xjm, pwolanin, joelpittet: Document SafeMarkup::set in Xss::filter

core/lib/Drupal/Component/Utility/Xss.php

@@ -79,6 +79,10 @@ public static function filter($string, $html_tags = array('a', 'em', 'strong', '
     $splitter = function ($matches) use ($html_tags, $class) {
       return $class::split($matches[1], $html_tags, $class);
     };
+    // Strip any tags that are not in the whitelist, then mark the text as safe
+    // for output. All other known XSS vectors have been filtered out by this
+    // point and any HTML tags remaining will have been deliberately allowed, so
+    // it is acceptable to call SafeMarkup::set() on the resultant string.
     return SafeMarkup::set(preg_replace_callback('%
       (
       <(?=[^a-zA-Z!/])  # a lone <