Suggest `var_names` when using deprecated API for partial traces

[lancechua]

Description

Sampling a partial trace was first done in #271, with the trace kwarg. Then, it subsequently removed in #6269 raising DeprecationWarning error. Partial trace was added back with the var_names kwarg in #7206.

This PR updates the error message to reference the new var_names kwarg instead, as trace may still be suggested by coding LLMs.

Related Issue

• [ ] Closes #
• [ ] Related to #271 #6269 #7206

Checklist

• [x] Checked that the pre-commit linting/style checks pass
• [ ] Included tests that prove the fix is effective or that the new feature works
• [ ] Added necessary documentation (docstrings and/or example notebooks)
• [ ] If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [ ] New feature / enhancement
• [ ] Bug fix
• [x] Documentation
• [ ] Maintenance
• [ ] Other (please specify):

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📚 Documentation preview 📚: https://pymc--7289.org.readthedocs.build/en/7289/

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[lancechua]

An alternative is var_names = trace if isinstance(trace, list) else var_names, but that would violate the one-way-to-do-it "rule"...

[ricardoV94]

Thanks @lancechua, this seems fine

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