DOC: elaborate on copies vs in place operations in comparison docs
<img width="782" alt="Screen Shot 2021-01-06 at 12 15 05 AM" src="https://user-images.githubusercontent.com/86842/103731907-437e7280-4fb4-11eb-9e6d-e702f117656c.png">

- [ ] ~~closes #xxxx~~
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] ~~whatsnew entry~~
