docs(dataframe): improve dataframe! and from_columns documentation

[cj-zhukov]

Is your feature request related to a problem or challenge?

Summary

Follow-up to the dataframe! macro PR #16090.

• Improve documentation for dataframe! and DataFrame::from_columns().
• Align their examples with the existing DataFrame documentation pattern.
• Use expected output assertions instead of commented DataFrame output.

Describe the solution you'd like

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Describe alternatives you've considered

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Additional context

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[cj-zhukov]

take