DOC: filter method for *GroupBy objects

[javidcf]

There does not seem to be a reference to the filter method of GroupBy/SeriesGroupBy/DataFrameGroupBy objects in the API documentation, even though it is mentioned in the Group By documentation.

[VincentLa]

I can pick this up! If anyone is at pycon sprints in portland, happy to pair on this as well.

[VincentLa]

Well actually it looks like the example is defined here for SeriesGroupBy and here for NDFrameGroupBy. So is the fix to this to just add it to api.rst?

#16484