Unexpected behavior of the str attribute which is working with Series of list
#### Code Sample, a copy-pastable example if possible

import pandas as pd
s1 = pd.Series(['AA/aa', 'BB/bb', 'CC/cc'])
s2 = s1.str.split('/')
s2.str[0]

Result:
0    AA
1    BB
2    CC
dtype: object

#### Problem description

In this example, the second 'str' attribute is applyied to a Series of list and not to a Series of string.
Then the [] operator works fine with each list an retrieve their first element...

As it is an unexpected working behavior, one may wonder if it is secure to code like this (instead of working with an apply + lambda for instance). This works also with Series of dict, and probably with any object implementing the [] operator.

#### Expected Output

Warning or error?
Or a word in the documentation?

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 69 Stepping 1, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.4
pytest: 3.3.2
pip: 19.0.1
setuptools: 38.4.0
Cython: 0.27.3
numpy: 1.14.2
scipy: 1.0.0
pyarrow: None
xarray: 0.10.9
IPython: 6.2.1
sphinx: 1.6.6
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.2
openpyxl: 2.5.12
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.2.3
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.1
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
