fixup! Add some (very) high-level docs for the data format

docs/data-storage.md

@@ -28,7 +28,7 @@ and HDF5 files.
 
 In v1 there were a few changes to the way we store images and `xarray` types:
 
-- RGBA images (including thumbnails) are stored as PNG byte arrays.
+- Thumbnails are stored as PNG byte arrays.
 - Support for `xarray.Dataset`'s was added.
 - `Dataset`'s and `DataArray`'s are stored inline in the NetCDF 4 format. This
   means we get support for saving all properties of `Dataset`'s/`DataArray`'s