Publish API for `Stdenv`.

`@private` hides classes from documentation altogether, while `@api
private` merely marks it as a private API.

Library/Homebrew/extend/ENV/std.rb

@@ -4,7 +4,7 @@
 require "hardware"
 require "extend/ENV/shared"
 
-# @private
+# @api private
 module Stdenv
   extend T::Sig