lib.literalExpression: add example and properly describe function arg

    is necessary for complex values, e.g. functions, or values that depend on

    other values or packages.

    # Examples

    :::{.example}

    ## `literalExpression` usage example

    ```nix

    llvmPackages = mkOption {

      type = types.str;

      description = ''

        Version of llvm packages to use for

        this module

      '';

      example = literalExpression ''

        llvmPackages = pkgs.llvmPackages_20;

      '';

    };

    ```

    :::

    # Inputs

    `text`

    : 1\. Function argument

    : The text to render as a Nix expression

  */

  literalExpression =

    text: