Update mask doc.
Update mask doc.

johanvdw/rasterio,perrygeo/rasterio,brendan-ward/rasterio,kapadia/rasterio,johanvdw/rasterio,njwilson23/rasterio,perrygeo/rasterio,clembou/rasterio,youngpm/rasterio,kapadia/rasterio,youngpm/rasterio,clembou/rasterio,perrygeo/rasterio,kapadia/rasterio,johanvdw/rasterio,njwilson23/rasterio,brendan-ward/rasterio,clembou/rasterio,njwilson23/rasterio,brendan-ward/rasterio,youngpm/rasterio