Document the behavior of idxmin when there are multiple rows containing the minimum value.
I've added an example to the docs which demonstrates the behavior of idxmin() for a column where multiple rows contain the minimum value (and also the column contains a NaN value).  The current documentation is ambigous (to me) on this edge case.

From http://pandas.pydata.org/, a pull request seemed like the most appropriate way to contribute to the docs.  Please let me know if there is a better way or how I can improve the request.  And thanks for a great tool!

Thanks! cherry-picked in
