[docs] Add Google-style docstrings for dspy/adapters/chat_adapter.py ChatAdapter class

[azai91]

What feature would you like to see?

I would like to improve docstring coverage for some of the public adapter APIs - starting with the ChatAdapter init method

Ref: #8926

Would you like to contribute?

• Yes, I'd like to help implement this.
• No, I just want to request it.

Additional Context

No response

[chenmoneygithub]

@azai91 Thank you for taking on this! Looking forward to your PR