function parseMarkersOption(markers) {
	// `*` is a marker for JSDoc comments.
	if (!markers.includes("*")) {
		return markers.concat("*");
	}

	return markers;
}
Parses `markers` option.
If markers don't include `"*"`, this adds `"*"` to allow JSDoc comments.
@param {string[]} [markers] A marker list.
@returns {string[]} A marker list.