DOC: Updating str_slice docstring
Added Examples to docstring.
Added documentation to each part of docstring
Hello @JesperDramsch! Thanks for submitting the PR.

 - In the file [`pandas/core/strings.py`](https://github.com/pandas-dev/pandas/blob/f9f8f8be3a6d7b31d0588daf1543fac275ddd038/pandas/core/strings.py), following are the PEP8 issues :

> [Line 1345:15](https://github.com/pandas-dev/pandas/blob/f9f8f8be3a6d7b31d0588daf1543fac275ddd038/pandas/core/strings.py#L1345): [W291](https://duckduckgo.com/?q=pep8%20W291) trailing whitespace
> [Line 1346:13](https://github.com/pandas-dev/pandas/blob/f9f8f8be3a6d7b31d0588daf1543fac275ddd038/pandas/core/strings.py#L1346): [W291](https://duckduckgo.com/?q=pep8%20W291) trailing whitespace

 I messed up. It continues here: #22569  FYI, you can always "fix" your PR with subsequent commits.  No need to create new PR's for each draft of your fixes if that makes things easier. @gfyoung thanks. I messed up worse though. Deleting a fork will orphan all PRs, something new I learned. 