DOC: Add validation error when a docstring is missing 
- [x] closes #23648
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @avolkov! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/base.py`](https://github.com/avolkov/pandas/blob/6a526cd2161b0da6d0770e4233eb5f7ed4d990a3/pandas/core/base.py) !

 - There are no PEP8 issues in the file [`pandas/core/generic.py`](https://github.com/avolkov/pandas/blob/6a526cd2161b0da6d0770e4233eb5f7ed4d990a3/pandas/core/generic.py) !

 - There are no PEP8 issues in the file [`pandas/core/series.py`](https://github.com/avolkov/pandas/blob/6a526cd2161b0da6d0770e4233eb5f7ed4d990a3/pandas/core/series.py) !

 - There are no PEP8 issues in the file [`scripts/tests/test_validate_docstrings.py`](https://github.com/avolkov/pandas/blob/6a526cd2161b0da6d0770e4233eb5f7ed4d990a3/scripts/tests/test_validate_docstrings.py) !

 - There are no PEP8 issues in the file [`scripts/validate_docstrings.py`](https://github.com/avolkov/pandas/blob/6a526cd2161b0da6d0770e4233eb5f7ed4d990a3/scripts/validate_docstrings.py) !



##### Comment last updated on November 13, 2018 at 16:28 Hours UTC New PR is fine. I'll close this one out