Autocomplete: Support for user-defined docstrings

For the current autocomplete plugin, user-defined functions defined in program scope do show up in the list of names presented to the user.
However, a nice-to-have would be to allow Python docstrings to serve as documentation for the function, similar to documentation for built-ins 

For example,
```py
def f(x):
    """
    Documentation for the `f` function
    """
    return x
```


    