DOC: improve groupby.DataFrameGroupBy.resample docstring
After asking for feedback, this is the very minimal _docstring_ that we interpret that would be acceptable. 

This is in order to unlock the full list of functions we have assigned for the pandas documentation sprint.

If there is something to be improved here please tell us. Also we may make a new push of this documentation including the full scope of the _args_ and _kwargs_ parameters and more examples.