Add informal definition of "label" in note
The term "label" is used extenisively, but (AFAICT) it is never explained what a "label" is. Is an integer offset a label? Do labels apply to rows and columns, multi-indexes? What's the relation between an "index" "axis" and "label" ... etc.

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