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.
Are you sure the `@todo api` annotation is needed? If the parent is exportable, then this one should be exportable too.