DOC: Harmonize column selection to bracket notation
As [suggested by](https://medium.com/dunder-data/minimally-sufficient-pandas-a8e67f2a2428#46f9) @tdpetrou. If there is agreement about not _presenting_ the dot notation to access columns in the docu, please let me know and I'll then check the rest of the docu.

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