DOC: Fix PR06 errors in Series docstrings (#28253)
First bit of work on #28253, fixes PR06 validation errors in Series docstrings.
Mostly just converting boolean -> bool, string -> str, integer -> ; re-worded a little where necessary.

- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Ran validate_docstrings.py and confirmed all PR06 issues for Series are removed, and that no additional validation errors are added by this change.
