CLN: Replace Appender and Substitution with simpler doc decorator
- [X] closes #30933 
- [x] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

A new decorator to handle docstring formatting. There is also an update for an existing case to show how it works.