Notes about unique vs cat.categories #12278
- [x] closes #12278
- [ ] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] adding notes about .unique() and .cat.categories (as suggested in [#12278 (comment)](https://github.com/pydata/pandas/issues/12278#issuecomment-182445522))

thanks @Dorozhko-Anton 
