Mistake in pd.DataFrame.assign() docstring

[toobaz]

The docstring for df.DataFrame.assign() starts with

Assign new columns to a DataFrame, returning a new object
(a copy) with all the original columns in addition to the new ones.

... but in fact existing columns can be overridden as well.

Hopefully tomorrow somebody will fix this!

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-5-amd64
machine: x86_64
processor:
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.23.0.dev0+463.g94ed47b15
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.7.0
Cython: 0.25.2
numpy: 1.14.1
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1

[HannahFerch]

I would like to take this on as my first open source contribution.

[toobaz]

@HannahFerch good idea! See https://pandas.pydata.org/pandas-docs/stable/contributing.html if you haven't already

[HannahFerch]

Thanks, this has been a great first experience!