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 think we've discovered this before actually.  Not sure whether to consider it a feature or a bug.  The JSDoc template used by the `generate-info.js` task doesn't do anything special with `@inheritDoc` annotations.  So when you override a method (and provide a doc block), you essentially clobber any annotations it would have otherwise acquired.

We could modify the template used by `generate-symbols.js` so symbols with `@inheritDoc` acquire `@api` annotations (as is done by the apidoc template for the stability level).  I'd like to see if we could push this to JSDoc itself though.

For now, I think we should add the extra `@api` annotation as you have.  Thanks for pointing this out @elemoine.