DOC: Series.idxmax actually returns a label, not an index
In https://github.com/pandas-dev/pandas/blob/master/pandas/core/series.py, the DocStrings for `idxmax` and `idxmin` are misleading (in my opinion).  They say they return the index of the max/min, but they actually return the label (if I've got my Pandas lingo right).

Same with `DataFrame.idxmax` and `DataFrame.idxmin`.  

Also, is there a reason `Series` provides `argmax` as a synonym for `idxmax`, and `DataFrame` does not?

One more thought: I find it surprising that `idxmax` and `argmax` do the same thing.  I expected `idxmax` to return an index and `argmax` to return a label.  I suppose it's too late to make that change.

