Change `BloomConfig` docstring
# What does this PR do?

This PR addresses small changes on the `BloomConfig` docstring that might be slightly confusing.
Original discussion from: https://huggingface.co/bigscience/bloom/discussions/120 

Thanks! 
cc @sgugger @VictorSanh @SaulLu 
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19336). All of your documentation changes will be reflected on that endpoint.