Document defaultProjection in JSON sources/formats

I'm documenting what the code currently does, but I have to say it doesn't seem very consistent to me. Why define this in both source and format? As it only applies to JSON formats, wouldn't it be better in format.JSONFeature, the parent of both GeoJSON and TopoJSON?

The TileVector example sets the defaultProjection in the format (though as 4326 is the default, I'm not sure this is necessary). defaultProjection is currently also defined as an option for TileVector source, as is 'object', but AFAICS these are never used, so I've removed them.