doc: Undocument attr-set of passthru.updateScript
doc: Undocument attr-set of passthru.updateScript

We no longer need it for most use cases so I am making it experimental.

I have something in mind where it might be useful in the future (customizing commit messages)
but for now, it would only confuse people.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs