Small documentation fix for MultiIndex.sortlevel
MultiIndex.sortlevel returns two values (MultiIndex and indices that sort the MultiIndex). Only the MultiIndex was described in the documentation. 

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15345?src=pr&el=h1) Report
> Merging [#15345](https://codecov.io/gh/pandas-dev/pandas/pull/15345?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/153da508a536a3ef203d9cd315c67b5fd3022a51?src=pr&el=desc) will **not impact** coverage.


```diff
@@           Coverage Diff           @@
##           master   #15345   +/-   ##
=======================================
  Coverage   86.32%   86.32%           
=======================================
  Files         141      141           
  Lines       51165    51165           
=======================================
  Hits        44169    44169           
  Misses       6996     6996
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15345?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/compare/153da508a536a3ef203d9cd315c67b5fd3022a51...c394448eb3d06c3aad08c2e2a49d0c931605e138?src=pr&el=tree#diff-cGFuZGFzL2luZGV4ZXMvbXVsdGkucHk=) | `96.58% <ø> (ø)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15345?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/15345?src=pr&el=footer). Last update [153da50...3a3a11a](https://codecov.io/gh/pandas-dev/pandas/compare/153da508a536a3ef203d9cd315c67b5fd3022a51...3a3a11a5a3d97a968dc6609691e621f7e6cddd53?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). this Returns is basically the same doc-string as ``.reindex``. can you make it like that formatting. thanks @J535D165 