mask based multi-index assignment of column values described
- [x] closes #34383
- [x] passes `black pandas`
not sure what this means, will look it up.
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`


- ~~[ ] tests added / passed~~ 
this is an PR on documentation only, unit tests are not needed.
- ~~[ ] whatsnew entry~~
I think I don't need this as this is a PR related to documentation only.

thanks @psteinb 