Clarify Js.xxx2 usage

Author: chenglou

pages/docs/manual/latest/api/js.mdx

@@ -35,6 +35,9 @@ Js.log("ReScript"->Js.String.startsWith("Re", _))
 Js.log(Js.String.split("-", "2019-11-10"))
 Js.log(Js.String.startsWith("Re", "ReScript"))
 ```
+## Js.Xxx2 Modules
+
+Prefer `Js.Array2` over `Js.Array`, `Js.String2` over `Js.String`, etc. The latters are old modules.
 
 ## Object
 

pages/docs/manual/v8.0.0/api/js.mdx

@@ -38,10 +38,7 @@ Js.log(Js.String.startsWith("Re", "ReScript"));
 
 ## Js.Xxx2 Modules
 
-For some modules with data-last argument order (e.g. [Js.String](./js/string)),
-there currently exists a matching module suffixed with "2" (e.g. [Js.String2](./js/string-2))
-that uses data-first argument order.
-These `Js.Xxx2` modules are non-public API, so their use is discouraged.
+Prefer `Js.Array2` over `Js.Array`, `Js.String2` over `Js.String`, etc. The latters are old modules.
 
 ## Object