docs/mkOption: improve mkOption documentation (#478096)

  isOption = lib.isType "option";

  /**

    Creates an Option attribute set. `mkOption` accepts an attribute set with the following keys:

    Creates an Option declaration for use with the module system.

    # Inputs

    Structured attribute set

    : Attribute set containing none or some of the following attributes.

    Attribute set

    : containing none or some of the following attributes.

      `default`

      : Optional default value used when no definition is given in the configuration.

      `readOnly`

      : Optional boolean indicating whether the option can be set only once.

      `...` (any other attribute)

      : Any other attribute is passed through to the resulting option attribute set.

    # Examples

    :::{.example}

    ## `lib.options.mkOption` usage example

    ```nix

    mkOption { }  // => { _type = "option"; }

    mkOption { default = "foo"; } // => { _type = "option"; default = "foo"; }

    mkOption { }

    # => Empty option; type = types.anything

    mkOption { default = "foo"; }

    # => Same as above, with a default value

    ```

    :::