(doc) update doc for helper defined from closure
(doc) update doc for helper defined from closure

wang19742008/handlebars-rust,tempbottle/handlebars-rust,sunng87/handlebars-rust,sunng87/handlebars-rust,ihrwein/handlebars-rust,tempbottle/handlebars-rust,tailhook/handlebars-rust,tailhook/handlebars-rust