DOC: filter method for *GroupBy objects
There does not seem to be a reference to the `filter` method of `GroupBy`/`SeriesGroupBy`/`DataFrameGroupBy` objects in the [API documentation](https://pandas.pydata.org/pandas-docs/stable/api.html), even though it is mentioned in the [Group By documentation](http://pandas.pydata.org/pandas-docs/stable/groupby.html#filtration).