wrappers: add meta.wrappers options to wrappers

Allows the docs to be a little less hard-coded.

Lays the groundwork for splitting up the platform-specific option docs
on a per-page basis.

wrappers/modules/darwin.nix

@@ -1,3 +1,7 @@
 {
   imports = [ ./shared.nix ];
+
+  config = {
+    meta.wrapper.name = "nix-darwin";
+  };
 }