Minor inaccurracy in documentation of read_csv's option mangle_dupe_cols
### Code example:

File `test.csv`:
```
,a,a,b
0,1,2,3
1,4,5,6
```

Python code:
``` python
import pandas as pd
df = pd.read_csv("test.csv")
df.columns.values
```

Gives `['a', 'a.1', 'b' ] ` and not, as documented `['a.0', 'a.1', 'b']`

#### Problem description

The documentation states that names will be specified as ‘X.0’...’X.N’, but in fact the names become 'X','X.1',...'X.N'

So in contrast to what the documentation says, the duplicate column-name is not changed at the first occurrence and only subsequent occurrences get a number appended.

#### Expected Output

Either change the code to mangle the first duplicate column name, or simply fix the documentation.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.14.11-200.fc26.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: 3.3.1
pip: 9.0.1
setuptools: 36.6.0
Cython: None
numpy: 1.14.0
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.5
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.3.0
numexpr: 2.6.1
feather: None
matplotlib: 2.1.1
openpyxl: None
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 1.0b10
sqlalchemy: 1.1.13
pymysql: None
psycopg2: 2.6.2 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
