Fix docstrings for vanilla clip
Author: isaac-chung
Pull request: yes

# What does this PR do?

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Fixes # (issue) https://github.com/huggingface/transformers/issues/26638 for `CLIPTokenizer`, `CLIPTokenizerFast`, `CLIPVisionConfig`


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      [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:
@ydshieh ready for ✅ thanks!

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_26691). All of your documentation changes will be reflected on that endpoint.