Fix type hint and documentation for new_fingerprint

[fxmarty]

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

datasets/src/datasets/fingerprint.py

Lines 446 to 454 in aa74388

	if inplace:
	new_fingerprint = update_fingerprint(self._fingerprint, transform, kwargs_for_fingerprint)
	else:
	for fingerprint_name in fingerprint_names: # transforms like `train_test_split` have several hashes
	if kwargs.get(fingerprint_name) is None:
	kwargs_for_fingerprint["fingerprint_name"] = fingerprint_name
	kwargs[fingerprint_name] = update_fingerprint(
	self._fingerprint, transform, kwargs_for_fingerprint
	)

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

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[lhoestq]

Good catch thanks ! LGTM :)