Improve ParameterizedFunction signature information

When trying to inspect the signature of a ParameterizedFunction I usually want to see the arguments as well as the parameters. However currently the signature only shows the parameters and lists *args and **kwargs in the signature. Here's an example from the datashader circular_layout function:

```
Init signature: circular_layout(*args, **params)
Docstring:
params(seed=Integer, uniform=Boolean, name=String)
```

This gives me no indication that the function expects nodes and edges as arguments.