Series.mode has not a `sort` argument.
#### Code Sample

```python
import pandas as pd

s = pd.Series(['b', 'c', 'a', 'c', 'a', 'b', 'a'])
s.mode(sort=False)

```

traceback:
```TypeError                                 Traceback (most recent call last)
<ipython-input-4-a5ccd47afe9c> in <module>()
      1 s = pd.Series(['b', 'c', 'a', 'c', 'a', 'b', 'a'])
----> 2 s.mode(sort=False)

TypeError: mode() got an unexpected keyword argument 'sort'
```

#### Problem description

Pandas DOC for `Series.mode` says that this function has a `sort` argument.
http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.mode.html

But looking at source code, I see that it is not true:
https://github.com/pandas-dev/pandas/blob/v0.19.2/pandas/core/series.py#L1220-L1237

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.8.0-46-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: ca_ES.UTF-8
LOCALE: None.None

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 35.0.0
Cython: 0.25.2
numpy: 1.12.0
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.3.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.7
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.1
openpyxl: 2.4.1
xlrd: 1.0.0
xlwt: None
xlsxwriter: 0.7.3
lxml: None
bs4: None
html5lib: 0.999999999
httplib2: None
apiclient: None
sqlalchemy: 1.0.14
pymysql: 0.7.5.None
psycopg2: 2.7.1 (dt dec pq3 ext lo64)
jinja2: 2.9.6
boto: None
pandas_datareader: None

</details>
