Improve option types documentation.

  # name (name of the type)

  # check (boolean function)

  # check (check the config value. Before returning false it should trace the bad value eg using traceValIfNot)

  # merge (default merge function)

  # iter (iterate on all elements contained in this type)

  # fold (fold all elements contained in this type)

      hasOptions = false;

    };

    # !!! this should be a type constructor that takes the options as

    # an argument.

    # usually used with listOf, attrsOf, loaOf like this:

    # users = mkOption {

    #   type = loaOf optionSet;

    #

    #   # you can omit the list if there is one element only

    #   options = [ {

    #     name = mkOption {

    #       description = "name of the user"

    #       ...

    #     };

    #     # more options here

    #   } { more options } ];

    # }

    # TODO: !!! document passing options as an argument to optionSet,

    # deprecate the current approach.

    optionSet = mkOptionType {

      name = "option set";

      # merge is done in "options.nix > addOptionMakeUp > handleOptionSets"