Fix docstring

The previous docstring was resulting in None

```
>>> class Foo:
...     """Parent class."""
...     
>>> class Bar(Foo):
...     Foo.__doc__ + \
...     """ Child class."""
...     
>>> Foo.__doc__
'Parent class.'
>>> Bar.__doc__
                    <---------- (None)
>>> class Bar2(Foo):
...     """ Child class."""
...     __doc__ = Foo.__doc__ + __doc__
...     
>>> Bar2.__doc__
'Parent class. Child class.'
```

This code still has some problem with -OO, resulting in:

```
TypeError: cannot concatenate 'str' and 'NoneType' objects
```