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.
@tschaub, @ahocevar, what do you think? Is it a bug in our build tools?