Doc build failing on master
Seems like https://travis-ci.org/pandas-dev/pandas/jobs/370527551 was the first one that failed.

```
Exception occurred:
  File "/tmp/doc/sphinxext/numpydoc/docscrape.py", line 375, in _error_location
    raise ValueError(msg)
ValueError: The section Returns appears twice in the docstring of None in None.
```