Clarify description of the is_split_into_words argument
# What does this PR do?
Clarifies the description of the `is_split_into_words` argument which is used in the tokenizers. 

Closes #11333 

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Issue: #11333 
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests? No additional tests were needed, I only clarified docs.


## Who can review?
I initially discussed this with @LysandreJik in #11333. I think @sgugger can review it too!

Thank you for taking the time to review this! 

NB the failed CI test seems unrelated.
Thank you for the feedback! 