CLN: a batch of replacements of @Appender() with @doc()
- [x] xref #31942
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] replaces a number of uses of @Appender() with @doc() in `core/generic.py`
