Resample docstring default values changed

This docstring seems better, since TimeGrouper specifies it [that way][1] too:

[1]: https://github.com/pydata/pandas/blob/master/pandas/tseries/resample.py#L27
Remove confusing default val hints from docstrings
Merge pull request #3085 from kermit666/patch-3

Resample docstring default values changed