pandas.DataFrame.unstack() documentation example has errors
The last part of the documentation says

``` python
>>> df = s.unstack(level=0)
>>> df.unstack()
one  a  1.
     b  3.
two  a  2.
     b  4.
```

although we'd have

``` python
>>> import pandas as pd
>>> index = pd.MultiIndex.from_tuples([('one', 'a'), ('one', 'b'),
...                                     ('two', 'a'), ('two', 'b')])
>>> import numpy as np
>>> s = pd.Series(np.arange(1.0, 5.0), index=index)
>>> s
one  a    1.0
     b    2.0
two  a    3.0
     b    4.0
dtype: float64
>>> s.unstack(level=-1)
       a    b
one  1.0  2.0
two  3.0  4.0
>>> s.unstack(level=0)
   one  two
a  1.0  3.0
b  2.0  4.0
>>> df = s.unstack(level=0)
>>> df.unstack()
one  a    1.0
     b    2.0
two  a    3.0
     b    4.0
dtype: float64
```
