Fix disable_nullable default value to False
Currently the `disable_nullable` parameter is not consistent across all dataset transforms. For example it is `False` in `map` but `True` in `flatten_indices`.

This creates unexpected behaviors like this
```python
from datasets import Dataset, concatenate_datasets

d1 = Dataset.from_dict({"a": [0, 1, 2, 3]})
d2 = d1.filter(lambda x: x["a"] < 2).flatten_indices()
d1.data.schema == d2.data.schema  # False
```
This can cause issues when concatenating datasets for example.

For consistency I set `disable_nullable` to `False` in `flatten_indices` and I fixed some docstrings

cc @SBrandeis 