DOC: Formatting for DtypeWarning Docstring [ci skip]
xref : https://github.com/pandas-dev/pandas/pull/20208

![fireshot capture 002 - pandas errors dtypewarning pandas 0_ - file____users_taugspurger_sandbox_](https://user-images.githubusercontent.com/1312546/37349929-958f175e-26a5-11e8-8c1e-872c2261c64f.png)

cc @jorisvandenbossche @hissashirocha
Stiill have the issue with the warning going to stdout. Here's the tail of the validation script.

```
No warning was issued.

>>> import os
>>> os.remove('test.csv')
scripts/validate_docstrings.py:268: DtypeWarning: Columns (0) have mixed types. Specify dtype option on import or set low_memory=False.
  runner.run(test)

################################################################################
################################## Validation ##################################
################################################################################

Errors found:
        No returns section found
``` For the validation script I think it is not that much of a problem. But for pytest, I think we don't want to generally capture and silence the warnings, as there might be others raised in docstrings that we want to get notified of.