Correct Create() api call description in Factory interface.

Remove reference to `path` in description and error list;
add format description for `id` string;
add "invalid format" error for `id` string;
remove initial capitals on references to parameter names;
remove reference to `initialConfig`.

I have deliberately chosen a restrictive “format description” for the `id` string, hoping that a better one will be forthcoming. Otherwise we can live with something like this (although whether Ω is a letter or not might vex some people).