bind-to-js-function.mdx: Clarify this binding documentation

Author: xtian

pages/docs/manual/latest/bind-to-js-function.mdx

@@ -376,7 +376,7 @@ x.onload = function (v) {
 
 </CodeTab>
 
-`this` has its first parameter is reserved for `this` and for arity of 0, there is no need for a redundant `unit` type.
+`@this` reserves the first parameter for `this`, and for arity of 0, there is no need for a redundant `unit` type.
 
 ## Function Nullable Return Value Wrapping