Make option docs evaluate

[roberth]

Required for hercules-ci/flake.parts-website#1070 to evaluate

Some defaults should probably be normal definitions or lib.mkDefault
definitions in the config section. They don't all make sense to be
documented defaults.

However, I've solved it with some less than optimal descriptions in
order not to move code around. Maybe I should have just moved it?

A few options are still missing descriptions, as shown by the build log:

nix build github:srid/flake.parts-website/rust-flake#checks.x86_64-linux.linkcheck --override-input rust-flake . --no-show-trace -vL
[...]
options.json> error: option perSystem.rust-project.crane-lib has no description
options.json> error: option perSystem.rust-project.crates.<name>.cargoToml has no description
options.json> error: option perSystem.rust-project.crates.<name>.crane.outputs.checks has no description
options.json> error: option perSystem.rust-project.crates.<name>.crane.outputs.packages has no description
options.json> error: option perSystem.rust-project.crates.<name>.path has no description

This PR is meant to get you across the initial bump of making it eval, so we can get the docs to build. Could you review and complete it? Feel free to push to my fork's branch.

Some general recommendations:

• Some definitions where a documented default isn't needed could be lib.mkDefault in config instead
• Please add a command like the above to CI (replacing srid's branch by upstream when merged)
• Always add a defaultText if the default isn't a constant literal / references any variable, option or function

[srid]

@roberth Appreciate this PR; sorry I had let it slip. Will merge soonish.

[roberth]

Don't worry about it :)