Fixed functions doc

You wouldn't want your user to use Twig's tag syntax (`{% %}`) for functions (`{{ }}`), as they would have some mysterious errors (`[LogicException] Could not process "xxx": "Unknown tag name "yyy" in "__string_template__zzz" at line aaa"`, `[Twig_Error_Syntax] Unknown tag name "yyy" in "__string_template__aaa" at line 3`)