mongodb3 - adds example of iterating over a DBCursor
The documentation says one can return a `DBCursor` but doesn't show an example how to iterate over it in order to work with the actual data. I found this way of doing it based on [this gist](https://gist.github.com/padewitte/1e159475b0ae7a9b6b8e) and thought future readers might appreciate the concrete example.
If the correct way to iterate is different I'll gladly update the PR or close this one if a different PR solves my case.
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