Koalas: Return type consistency in docs

[sorenmc]

It is currently not transparent what the return type of a kolas dataframe / series is.

Take for example
DataFrame.max()

Returns
maxscalar for a Series, and a Series for a DataFrame.

Returned type i receive is a pandas.Series

Series.value_counts()

Returns
counts Series

Returned type i receive is a databricks.koalas.series.Series

I would like to suggest that it is made clear in the docs across all functions what the return type is in all cases

In both these cases it is unclear what type of series is returned

[HyukjinKwon]

I think we should match it to Koalas Series.

[sorenmc]

kdf.nunique() still returns a pandas series

[HyukjinKwon]

Thanks for letting me know. Will fix soon