[docs] Clarify Parquet doc
Addresses this [comment](https://github.com/huggingface/blog/pull/1177#pullrequestreview-1465226825) to clarify the requirements for generating a Parquet dataset.
[docs] Clarify Parquet doc: Addresses this [comment](https://github.com/huggingface/blog/pull/1177#pullrequestreview-1465226825) to clarify the requirements for generating a Parquet dataset.