the doc for overlay is a WIP but it contains a section on overlays doc: update lua documentation by teto · Pull Request #55302 · NixOS/nixpkgs · GitHub .
Alternatively you can lookup https://github.com/teto/home/blob/af268c23b2ab4ac1c2d8a0e40daeca81cb320268/config/nixpkgs/overlays/lua.nix