Use inheritDoc for ol.source.XYZ#setTileUrlFunction

And make that method exportable. (Note that the parent method is already [exportable](https://github.com/openlayers/ol3/blob/master/src/ol/source/tileimagesource.js#L145).)

fixes #2249.
I also ran `generate-info.js` as suggested by @tschaub. It confirmed the problem. Without the `@todo api` annotation `ol.source.XYZ#setTileUrlFunction` is not present in the generated `info.json` file. And it is present if I add the annotation.