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.
So with just `@inheritDoc` and without `@todo api` I get the following in the console in the (hosted) xyz example:

```
> map.getLayers().item(1).getSource().setTileUrlFunction
function (a){this.c.clear();this.tileUrlFunction=a;this.n()}
```

Now if I add `@todo api` (as in my patch) I get this:

```
> map.getLayers().item(1).getSource().setTileUrlFunction
function (a){zw.L.ia.call(this,Dv(this.e,a))} 
```

So this shows that without `@todo api` the `ol.source.XYZ#setTileUrlFunction` property is not exported.

It sounds like that is not what you would expect…