fix `word_to_tokens` docstring format
Author: SaulLu
Pull request: yes

# What does this PR do?

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This PR fix a format issue in the `word_to_tokens` docstring which prevented the method outputs from being displayed on the documentation site.

I also added an example for the None value returned.

Fix https://github.com/huggingface/transformers/issues/20449

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


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Comments:
_The documentation is not available anymore as the PR was closed or merged._