(DOC) A `string` passed to `groupby` is hard to understand based on current doc
#### Code Sample, a copy-pastable example if possible
From [Here](pandas/doc/source/groupby.rst)
```rst
For DataFrame objects, a string indicating a column to be used to group. Of course 
df.groupby('A') is just syntactic sugar for df.groupby(df['A']), but 
it makes life simpler
For DataFrame objects, a string indicating an index level to be used to group.

```
#### Problem description

These two sentences are in a kind of conflict with each other, until one read until she read the note below.
#### Expected Output
Reword to make it clear that a string may indicate column or index level

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Darwin
OS-release: 16.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.0.dev+193.gb2b5dc32e
pytest: 3.1.2
pip: 9.0.1
setuptools: 36.0.1
Cython: 0.25.2
numpy: 1.13.0
scipy: 0.19.0
xarray: None
IPython: 6.0.0
sphinx: 1.6.2
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: None
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
