Consider removing references to "needCol" and explaining what "rand" is in GetRowCursor

[pgovind]

I realize this is still in preview, but I'm just putting a note here since I just spent an hour or so referring to this. Feel free to ignore if this is too early

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Document Details

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• ID: cebd0074-01d3-d048-305a-cfea8b4083de
• Version Independent ID: 7f1aaf72-e345-e334-6237-d92abd3aa6d3
• Content: IDataView Interface (Microsoft.ML)
• Content Source: dotnet/xml/Microsoft.ML/IDataView.xml
• Product: dotnet-ml-api
• GitHub Login: @sfilipi
• Microsoft Alias: johalex