DOC: Rephrased doc for Series.asof. Added examples
Used code from https://github.com/pandas-dev/pandas/issues/20652 as example, to illustrate different behaviours.

- [x] closes #20652
- [x] tests added / passed (N/A, docs)
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
