Fix type hint and documentation for `new_fingerprint`
_The documentation is not available anymore as the PR was closed or merged._
Currently, there are no type hints nor `Optional` for the argument `new_fingerprint` in several methods of `datasets.arrow_dataset.Dataset`.

There was some documentation missing as well.

Note that pylance is happy with the type hints, but pyright does not detect that `new_fingerprint` is set within the decorator.

The modifications in this PR are fine since here https://github.com/huggingface/datasets/blob/aa743886221d76afb409d263e1b136e7a71fe2b4/src/datasets/fingerprint.py#L446-L454

for the non-inplace case we make sure to auto-generate a new fingerprint (as indicated in the doc).