ENH: merge set validation (#29649)
Added new feature to pandas merge to validate "set-theory" relationship
between left and right dataframes. Updated docs and tests for new
feature.

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