If you want to be able to allow people to call some methods using None you have to do use a sentinel object when you define the method.
None
_senti
The part of generated method signature can be changed by overriding the __repr__ method of the sentinel object.
__repr__
_sentinel = type('_sentinel', (object,), {'__repr__': lambda self: '_sentinel'})()
It will be rendered by Sphinx as something like this:
mymodule.foo(param1=_sentinel)