Carry a codec's comments into the option it generates
A comment written with `<?>` or `<??>` was thrown away when a field became a Nix option, so only the field's own description survived into `lib.mkOption`.
That is the wrong half to keep for a shared type. A field says what it is for where the field is; a type says what it means where the type is. Whoever reads the NixOS module generated from the codec wants both, and documentation written on a codec reached the rendered configuration documentation while silently never reaching the module.
## What changed
`objectCodecNixOptions` now builds each option's description from the field's own description followed by the comments on the codec under it, outermost first, joined by newlines.
```haskell
requiredFieldWith "issuer" (codec <?> "more about it") "the issuer"
```
```nix
issuer = lib.mkOption {
description = "the issuer\nmore about it";
type = lib.types.str;
};
```
It descends through a `BimapCodec`, as `valueCodecNixOptionType` already does, because a codec written for one type in terms of another is still that type's codec. It does not descend into an `EitherCodec` or an `ArrayOfCodec`, whose comments are about something the option is made of rather than about the option itself.
## Compatibility
Generated output changes for any codec that has a comment on a field's value codec, so downstream golden files move. Nothing in this repository's own goldens changed, which is why the new `CommentedFields` fixture exists: it covers a required field, an optional one, one with an omitted default, and one with a comment and no description of its own.
A field with a comment and no description now has a description where it had none at all.
`autodocodec-nix` goes to 0.1.1.0.