[QST] Clarify when `agg()` will return groups as index
```
In [44]: a = pd.DataFrame({'a': [1, 1, 2], 'b': [1, 2, 3]})

In [45]: a.groupby('a').agg('sum') # 'a' is returned as index
Out[45]:
   b
a
1  3
2  3

In [46]: a.groupby('a').agg('nunique') # 'a' is returned both as index and column
Out[46]:
   a  b
a
1  1  2
2  1  1
```

The groupby [documentation](https://pandas.pydata.org/pandas-docs/stable/user_guide/groupby.html#aggregation) notes:

> Aggregation functions will not return the groups that you are aggregating over if they are named columns, when as_index=True, the default. The grouped columns will be the indices of the returned object.

However, in the snippet above, it looks like the `nunique` aggregation behaves differently in this respect from the `sum` aggregation.

Is it possible to predict when an aggregation will return groups as index columns v/s a "SQL-style output" v/s a combination of the two (as above)?