Additional DOC and BUG fix related to merging with mix of columns and…
Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

- [x] closes #20452
- [x] tests added / passed
   - test_join.py 
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
   - added to v0.24 related to bug fix

This PR adds some documentation about how `MultiLevel` index levels are dropped when doing a merge that includes levels and columns.  It also fixes a small bug related to `join`.
