Resample docstring default values changed
This docstring seems better, since TimeGrouper specifies it [that way](https://github.com/pydata/pandas/blob/master/pandas/tseries/resample.py#L27) too:

I'd simply remove the default val description from Timegrouper, 
the real behaviour is too much info for a docstring.
 Is this OK now or should I change something else in the docstring?
 merged.
 Great, thanks!
