DOC: whatsnew note for MultiIndex Sorting Fix
xref https://github.com/pandas-dev/pandas/pull/21043#issuecomment-394342149
Should this be in the 0.24.0 whatsnew instead?  The xref'd PR has a 0.24.0 milestone.  Not opposed to this going in 0.23.1, just want to make sure this is aligned with the xref'd PR. Good question. Since it's a bug fix I am assuming v0.23.1 but can move if we want # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21316?src=pr&el=h1) Report
> Merging [#21316](https://codecov.io/gh/pandas-dev/pandas/pull/21316?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b32fdc44206c38aecbbe5fdb4ed543a5d213ebb9?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21316/graphs/tree.svg?height=150&src=pr&width=650&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21316?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21316   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files         153      153           
  Lines       49549    49549           
=======================================
  Hits        45512    45512           
  Misses       4037     4037
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.25% <ø> (ø)` | :arrow_up: |
| #single | `41.87% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21316?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21316?src=pr&el=footer). Last update [b32fdc4...6235bea](https://codecov.io/gh/pandas-dev/pandas/pull/21316?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 OK, the fix seems rather small and it fixes an actual data corruption, so let's target it as 0.23.1