Document the difference between resources.completions and resources.chat.completions

[mmacy]

Confirm this is a feature request for the Python library and not the underlying OpenAI API.

• This is a feature request for the Python library

Describe the feature or improvement you're requesting

Neither the module docstrings (missing) nor class docstrings (missing or anemic) make it clear how these differ:

• openai.resources.completions
• openai.resources.chat.completions

Additional context

The list of language models is different in their create() methods, but don't make the dev infer or guess which one they should use for what - tell 'em right there in the docstring so they can see it when they hover their mouse over the attribute in their IDE.

[mmacy]

Fixed by #5