DataFrame.stack() DataFrame.unstack() documentation/behavior
`DataFrame.stack()` and `DataFrame.unstack()` methods require docstring update.

`DataFrame.stack()` docstring mentions that method converts DataFrame to Series, and returns a Series. This is only the case when _columns_ has only one level. In case _columns_ has multiple levels, method returns a DataFrame.

`DataFrame.unstack()` docstring is mainly a copy of the `Series.unstack()` docstring, example should operate on a DataFrame and not a Series.
Concerning the behavior, when the DataFrame index is a single level one, _unstack()_ raises: 

``` python
AttributeError: `Index` object has no attribute `levels`
```

`DataFrame.unstack()` assumes a DataFrame with a multilevel index, it can not operate on single level index, maybe the method could return a Series in case _unstack()_ operates on DataFrame with single level index, similar as _stack()_ does in case of single level _columns_?
