Improve python documentation

[matthewmturner]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
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As a user of the datafusion python bindings i would like better documentation and guides on how to use the python bindings

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1. Add class / function / method signatures to https://arrow.apache.org/datafusion/python/api.html
2. Add class / function / method descriptions
3. Add class / function / method code examples

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[Jefffrey]

I'll close this as it is old and the Python API is in https://github.com/apache/datafusion-python repository now; feel free to re-open there if current documentation isn't sufficient