[DOCS] Add descriptive docstring to MinNewTokensLength
# What does this PR do?
It addresses one of the arguments in https://github.com/huggingface/transformers/issues/24783

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?
@gante


The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25196). All of your documentation changes will be reflected on that endpoint.<|||||>cc @gante <|||||>> Thank you for iterating raised_hands

Always a pleasure, thanks for the opportunity :hugs: 