DOC: Capitalize docstring summaries
Fixes #23881

- [x] closes #23881
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @nixphix! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/frame.py`](https://github.com/nixphix/pandas/blob/e4153254f1d1af6caed06c46fb73a55999c1d817/pandas/core/frame.py) !

 - There are no PEP8 issues in the file [`pandas/core/groupby/groupby.py`](https://github.com/nixphix/pandas/blob/e4153254f1d1af6caed06c46fb73a55999c1d817/pandas/core/groupby/groupby.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/base.py`](https://github.com/nixphix/pandas/blob/e4153254f1d1af6caed06c46fb73a55999c1d817/pandas/core/indexes/base.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/multi.py`](https://github.com/nixphix/pandas/blob/e4153254f1d1af6caed06c46fb73a55999c1d817/pandas/core/indexes/multi.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/range.py`](https://github.com/nixphix/pandas/blob/e4153254f1d1af6caed06c46fb73a55999c1d817/pandas/core/indexes/range.py) !

 - There are no PEP8 issues in the file [`pandas/core/resample.py`](https://github.com/nixphix/pandas/blob/e4153254f1d1af6caed06c46fb73a55999c1d817/pandas/core/resample.py) !

 - There are no PEP8 issues in the file [`pandas/core/window.py`](https://github.com/nixphix/pandas/blob/e4153254f1d1af6caed06c46fb73a55999c1d817/pandas/core/window.py) !

 - There are no PEP8 issues in the file [`pandas/io/json/normalize.py`](https://github.com/nixphix/pandas/blob/e4153254f1d1af6caed06c46fb73a55999c1d817/pandas/io/json/normalize.py) !

 - There are no PEP8 issues in the file [`pandas/io/pytables.py`](https://github.com/nixphix/pandas/blob/e4153254f1d1af6caed06c46fb73a55999c1d817/pandas/io/pytables.py) !

 Closing as redundant effort #23886