text: Title: [DOC] `groupby_agg` naming and documentation
  Body: `groupby_agg` is possible to implement with method-chaining in native pandas, contrary to what the documentation seems to imply; native pandas would use `groupby(...).transform`, e.g.
  ```python
  df = df.assign(new_column=lambda x: x.groupby("col")["col_to_agg"].transform(agg_func))
  ```
  Of course, this is a mouthful to type, so it still makes sense to have a function that takes care of this operation, but `groupby_agg` seems like the wrong name for the function, and something like `groupby_transform_new_column` would be more descriptive. 
  
  I'm not sure whether the implementation should also change to reflect that native pandas supports `groupby(...).transform`, and so the `.merge` would not be necessary.
  