Adds comment to clarify tricky preg check

Author: michalbundyra

src/Uri.php

@@ -603,6 +603,7 @@ private function filterPath(string $path) : string
      */
     private function filterInvalidUtf8(string $string) : string
     {
+        // check if given string contains only valid UTF-8 characters
         if (preg_match('//u', $string)) {
             return $string;
         }