DOC: Rewritten style.py docstrings: #12148
Docstrings for pandas/io/formats/style.py now accurately reflects the behavior of the ``Styler`` class. Fixed a typo in the docstring of the `style` property of ``DataFrame``.

- [x] closes #12148 
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
