Clarify/Improve EvalType for grouped aggregate and window aggregate

Currently, grouped aggregate and window aggregate uses different EvalType, however, they map to the same user facing type PandasUDFType.GROUPED_MAP.

It makes sense to have one user facing type because it (PandasUDFType.GROUPED_MAP) can be used in both groupby and window operation.

However, the mismatching between PandasUDFType and EvalType can be confusing to developers. We should clarify and/or improve this.

See discussion at: https://github.com/apache/spark/pull/22620#discussion_r222452544