DOC/PY3: np.unique errors on pd.Series with mixed string/float in python3
In a similar fashion as #6212, the following fails on Python 3 (`doc/source/reshaping.rst`):

``` python
import numpy as np
import pandas as pd
x = pd.Series(['A', 'A', np.nan, 'B', 3.14, np.inf])
np.unique(x, return_inverse=True)[::-1])
```

```
/Users/user/.virtualenvs/pandas-dev/lib/python3.3/site-packages/numpy/lib/arraysetops.py in unique(ar, return_index, return_inverse)
    181             perm = ar.argsort(kind='mergesort')
    182         else:
--> 183             perm = ar.argsort()
    184         aux = ar[perm]
    185         flag = np.concatenate(([True], aux[1:] != aux[:-1]))

TypeError: unorderable types: str() > float()
```

Although this error manifests in `numpy` code, I assume this has to do with `pd.Series`?
