Add attribute descriptions for `GenerateParameters` (#1798)
Once https://github.com/huggingface/huggingface.js/pull/629 gets merged,
we will rely on TGI's specs to generate jsonschema types for
`text_generation` and `chat_completion`.
This PR adds some documentation for `GenerationParameters`'s properties
so that they get documented in the downstream tools (TGI docs but also
`huggingface.js`/`huggingface_hub` inference clients). I mostly took
inspiration from [the python
client](https://github.com/huggingface/text-generation-inference/blob/main/clients/python/text_generation/types.py)
for the descriptions.