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.
This should be unnecessary.  Checking the `build/info.js` output (from `node tasks/generate-info.js`) with and without the annotation would give a definitive answer.  The `ol.source.XYZ#setTileUrlFunction` should be present without this annotation if it is present on a parent.