[red-knot] add TODO comment in specialization code (#17615)
## Summary
As promised, this just adds a TODO comment to document something we
discussed today that should probably be improved at some point, but
isn't a priority right now (since it's an issue that in practice would
only affect generic classes with both `__init__` and `__new__` methods,
where some typevar is bound to `Unknown` in one and to some other type
in another.)
