Correct docstrings of `BasePandasDataset`

[alexander3774]

Currently docstrings of DataFrame inherited from BasePandasDataset are not completely correct, for example DataFrame.abs:

python -c "import modin.pandas as pd; print(pd.DataFrame.abs.__doc__)"

        Return a `BasePandasDataset` with absolute numeric value of each element.

        Notes
        -----
        See pandas API documentation for `pandas.DataFrame.abs <https://pandas.pydata.org/pandas-docs/version/1.4.2/reference/api/pandas.DataFrame.abs.html>`_, `pandas.Series.abs <https://pandas.pydata.org/pandas-docs/version/1.4.2/reference/api/pandas.Series.abs.html>`_ for more.

Docstring denotes that BasePandasDataset is returned, when DataFrame actually returned.

Probably we should replace BasePandasDataset -> Series/DataFrame in the BasePandasDataset docstrings like pandas does for NDFrame.