`nix.settings.preallocate-contents` is not enabled by default

[DaniD3v]

Issue description

The documentation for states that this option decides

Whether to preallocate files when writing objects with known size.

This sounds like it should be enabled by default.

Are there any downsides to enabling this?
In that case they should definitely be documented.

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 6.6.39, NixOS, 24.05 (Uakari), 24.05.20240712.f12ee5f`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.18.4`
 - channels(root): `""`
 - nixpkgs: `/nix/store/2hmc93ki0rvz6d20lmbscqw32612p8an-source`