DOC/API: clarify groupby sorting behavior
clarify / doc / test (explicity) on passing `sort=True|False` to a `groupby/resample`
from [SO](http://stackoverflow.com/questions/26456125/python-pandas-is-order-preserved-when-using-groupby-and-agg/26465555#26465555resample forces a sort)

groupby only sorts if sort is True and the index is NOT monotonic.

just like to test / validate and write a doc not about this
