operator `*_notify` argument is not a constructor

[AndreasPlt]

While trying to write a custom operator using unary_notify, I noticed that - unlike unary and unary_frontier -, the closure argument given to the operator directly represents the operator logic itself, and not the constructor for the logic. The same holds for the binary version of the operators.

I haven't worked with timely for a long time, but if I understand correctly, this prevents me from incorporating state into the unary_notify logic. While the *_notify functions are just simple convenience wrappers for *_frontier, it would still be nice to have the option to include state in this logic. If this is not intended, then at least the docstrings for the *_notify functions should be edited to not mention a constructor argument.