refs #85: improve docs for MBXRasterTileOverlay
refs #85: improve docs for MBXRasterTileOverlay

mapbox/mbxmapkit,MadeByHecho/mbxmapkit,pixio/mbxmapkit,pixio/mbxmapkit,MadeByHecho/mbxmapkit,mapbox/mbxmapkit,MadeByHecho/mbxmapkit