DOC: Update Series.mode docstring
 - [ ] closes #xxxx
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry

Series mode method does not support sort parameter, even though the documentation says so. Neither does the underlying function from pandas.core.algorithms.
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14653?src=pr) is 85.28% (diff: 100%)

> Merging [#14653](https://codecov.io/gh/pandas-dev/pandas/pull/14653?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

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> Powered by [Codecov](https://codecov.io?src=pr). Last update [1d6dbb4...0d250c2](https://codecov.io/gh/pandas-dev/pandas/compare/1d6dbb41b26a39121ec8c4f19f5da78bb0ab4af7...0d250c22bf162f426583adfd29c7c1594a5d81ea?src=pr)
 @sakkemo Thanks for the PR!

This was added in the original PR that added the `mode` method, but this indeed does not really seem to be correct (https://github.com/pandas-dev/pandas/pull/5380). Possibly the return values will be sorted if there are multiple return values?
 Yes, the return values are sorted! It says so in the return value part of the docstring (hidden by the fold in github by default), but perhaps it could be added in the description as well.

```
     Returns
     -------
     modes : Series (sorted)
```

Regarding the list at the top, do I need to add an issue? Also I didn't know to which whatsnew file add the changes, 0.19.2 or 0.20.0. It depends on when this is merged, of course.
 @sakkemo Thanks!
