DOC: docstring for min max should point out idxmax and idxmin

[michaelaye]

Numpy users might be puzzled by the fact that everything is so numpy-like, but argmax and argmin are not implemented (of course having not read the manual), so I would like to suggest to point out idxmax and idxmin in the docstring of Series/DataFrame.max/min?

[jreback]

closed by #3580