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`.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20475?src=pr&el=h1) Report
> Merging [#20475](https://codecov.io/gh/pandas-dev/pandas/pull/20475?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/940104efc9e708bc93744dfaa36c9492b03b1ca4?src=pr&el=desc) will **increase** coverage by `49.74%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20475/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20475?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #20475       +/-   ##
===========================================
+ Coverage   42.51%   92.25%   +49.74%     
===========================================
  Files         161      161               
  Lines       51691    51692        +1     
===========================================
+ Hits        21975    47690    +25715     
+ Misses      29716     4002    -25714
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.66% <100%> (?)` | |
| #single | `42.51% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20475?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/20475/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.28% <100%> (+84.55%)` | :arrow_up: |
| [pandas/core/computation/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/20475/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vcHl0YWJsZXMucHk=) | `92.37% <0%> (+0.3%)` | :arrow_up: |
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/20475/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `92.3% <0%> (+0.92%)` | :arrow_up: |
| [pandas/util/\_test\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/20475/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX3Rlc3RfZGVjb3JhdG9ycy5weQ==) | `93.24% <0%> (+4.05%)` | :arrow_up: |
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/20475/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `58.36% <0%> (+8.17%)` | :arrow_up: |
| [pandas/core/config\_init.py](https://codecov.io/gh/pandas-dev/pandas/pull/20475/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnX2luaXQucHk=) | `99.24% <0%> (+9.84%)` | :arrow_up: |
| [pandas/core/reshape/util.py](https://codecov.io/gh/pandas-dev/pandas/pull/20475/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS91dGlsLnB5) | `100% <0%> (+11.53%)` | :arrow_up: |
| [pandas/compat/numpy/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/20475/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9udW1weS9fX2luaXRfXy5weQ==) | `92.85% <0%> (+14.28%)` | :arrow_up: |
| [pandas/core/computation/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/20475/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vY29tbW9uLnB5) | `85.71% <0%> (+14.28%)` | :arrow_up: |
| [pandas/core/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/20475/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBpLnB5) | `100% <0%> (+14.81%)` | :arrow_up: |
| ... and [120 more](https://codecov.io/gh/pandas-dev/pandas/pull/20475/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20475?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20475?src=pr&el=footer). Last update [940104e...d0b6aed](https://codecov.io/gh/pandas-dev/pandas/pull/20475?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 you showed 3 examples in the OP, can you replicate them here @jreback I made the changes you asked for and it's all green @Dr-Irv this got a bit lost, can you rebase and update Hello @Dr-Irv! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/reshape/merge.py`](https://github.com/Dr-Irv/pandas/blob/0752e59de7cdcc8c23bf78a577ebcd03f49dd94a/pandas/core/reshape/merge.py) !

 - There are no PEP8 issues in the file [`pandas/tests/reshape/merge/test_join.py`](https://github.com/Dr-Irv/pandas/blob/0752e59de7cdcc8c23bf78a577ebcd03f49dd94a/pandas/tests/reshape/merge/test_join.py) !

 - There are no PEP8 issues in the file [`pandas/tests/reshape/merge/test_merge.py`](https://github.com/Dr-Irv/pandas/blob/0752e59de7cdcc8c23bf78a577ebcd03f49dd94a/pandas/tests/reshape/merge/test_merge.py) !

 @jreback merged with latest, and now all green.  I now added a line in whatsnew. @Dr-Irv sorry this wasn't merged before, can you do one more rebase please? @jreback  @datapythonista now all green Sorry @Dr-Irv, another rebase needed. @datapythonista I've done the merge, but the failure is because there is something wrong on Azure.  Up to you to decide how to proceed. @datapythonista Current merge is now all green. lgtm. @Dr-Irv just pushed a commit to fix whitespace. ping on green. thanks @Dr-Irv 