Changing the docs to be friendlier.
The implementation of `allow_none` for compound types was checking the wrong
variable for the `serialize_when_none` setting.

This is now working correctly.

    >>> class Foo(Model):
    ...     s = StringType()
    ...     class Options:
    ...         serialize_when_none = True
    ...
    >>> f = Foo()
    >>> f.serialize()
    {'s': None}