Upon further thought, not a problem. Stephen considered the case:
class myint(int):
    # inherit __init__
    def added_func(self): "some new function of an int"

In this case, fob = myint.__init__ would not be FunctionType and inspect would not be called. Tool tip would be myint.__doc__ and writer could follow style of builtins. Still an interesting test case to add.

A different question is whether the default for callables (as opposed to non-callables) should be nothing or something like 'args unknown' or 'see docs'.