Update WhisperTokenizer Doc: Timestamps and Previous Tokens Behaviour
Author: bruno-hays
Pull request: yes



# What does this PR do?
https://github.com/huggingface/transformers/issues/32378
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@ylacombe

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Comments:
Thanks @ylacombe !
I ran into some issues running `make fixup` due to it needing both torch and tensorflow.
But I get this before it crashes:
```
Checking/fixing src/transformers/models/whisper/tokenization_whisper.py src/transformers/models/whisper/tokenization_whisper_fast.py
All checks passed!
2 files left unchanged
python utils/custom_init_isort.py
python utils/sort_auto_mappings.py
python utils/check_doc_toc.py --fix_and_overwrite
running deps_table_update
updating src/transformers/dependency_versions_table.py
python utils/check_copies.py
python utils/check_table.py
python utils/check_dummies.py
python utils/check_repo.py
```

I hope it means it's ok, no file changed

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