Add docstrings to dispatched functions


```Python
@dispatch(object, object)
def f(x, y):
    """ Docstring number one """ 
    return x + y

@dispatch(int, int)
def f(x, y):
    """ Docstring number two """
    return x + y
    
@dispatch(float, float)
def f(x, y):
    return x + y

>>> f?
 Multiply dispatched method: f

Inputs: <object, object>
-------------------------
Docstring number one

Inputs: <int, int>
-------------------
Docstring number two

Other signatures:
    float, float

```