DOC: Do not mention private classes in the documentation
The public documentation shouldn't mention private classes, which the users are not expected to know about, and they could be removed in the future without proper deprecation.

We've got a script to find them, it can be run with `./scripts/validate_docstrings.py --errors=GL04`

Currently there are those cases:
```
pandas.Series.pipe: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.Series.where: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.Series.mask: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.Series.first_valid_index: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.Series.last_valid_index: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.Series.tshift: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.Series.to_dense: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.core.resample.Resampler.get_group: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.DataFrame.where: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.DataFrame.mask: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.DataFrame.pipe: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.DataFrame.tshift: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.DataFrame.first_valid_index: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.DataFrame.last_valid_index: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.DataFrame.to_dense: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.core.groupby.GroupBy.get_group: Private classes (NDFrame) should not be mentioned in public docstrings
pandas.core.groupby.DataFrameGroupBy.tshift: Private classes (NDFrame) should not be mentioned in public docstrings
```

I'd say in most cases we can replace `NDFrame` by `Series or DataFrame` but may be we can rephrase sentences too.

After all them are corrected, we should add to `ci/code_checks.sh` the code `GL04` to the part where we validate the documentation, so the CI checks for it, and mentions to `NDFrame` or other private classes are not reintroduced. Also in the same PR we can add `GL05`, which is not validated but there are no errors in the code for it, so we can start validating it.