[Dataset.to_dataframe](https://github.com/pydata/xarray/blob/1be777fe725a85b8cc0f65a2bc41f4bc2ba18043/xarray/core/dataset.py#L4536) should have a return type hint like [DataArray.to_dataframe](https://github.com/pydata/xarray/blob/1be777fe725a85b8cc0f65a2bc41f4bc2ba18043/xarray/core/dataarray.py#L2368). Similarly, can [concat](https://github.com/pydata/xarray/blob/1be777fe725a85b8cc0f65a2bc41f4bc2ba18043/xarray/core/concat.py#L11) have a `Union[Dataset, DataArray]` return type or is it more complicated than that?