Document how to use specific `all-cabal-hashes`

[deemp]

The #197 (comment) provides a solution how to override all-cabal-hashes.

Some other args can be overridden https://github.com/NixOS/nixpkgs/blob/62518b5aafd1c296bb86bc270eb300f29ca45300/pkgs/development/haskell-modules/default.nix. Perhaps some of them should also be mentioned in the docs.