DOC: Capitalize docstring summaries
- [x] closes #23881
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] Solves most of the issues raised by the `validate_docstrings.py` script. 

The few issues that still show up on running the script can be attributed to- 
- Docstrings non-existent viz. `tseries.offsets.Tick`, `tseries.offsets.Day` etc. (working on this for a separate PR)
- Auto-generated Doctrings viz. `Series.argmin`, `Timestamp.strptime` etc.
- Docstrings starting with quotes viz. 'Normalizes', 'Unpivots' which were (presumably) added in by the original authors by for a reason.
