DOC: clarify boxplot whiskers extension
As I'm getting started with box plots, I found confusing that the `DataFrame.boxplot` docstring didn't mention that whiskers extend _at most_ to 1.5 * IQR away from the box's edges.

- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry -> is it required for such a minor change ?
