Updated docs for the BM25 preprocessing function (#28101)
- [x] **PR title**: "docs: add explanation for preprocessing function"
- [x] **PR message**:
- **Description:** Extending the BM25 description and demonstrating the
preprocessing function
- **Dependencies:** nltk
- **Twitter handle:** @kirilbuga
@efriis
@baskaryan
@vbarda
@ccurme
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Co-authored-by: Chester Curme <chester.curme@gmail.com>
