[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.)

crates/red_knot_python_semantic/src/types/generics.rs

@@ -154,6 +154,10 @@ impl<'db> Specialization<'db> {
     pub(crate) fn combine(self, db: &'db dyn Db, other: Self) -> Self {
         let generic_context = self.generic_context(db);
         assert!(other.generic_context(db) == generic_context);
+        // TODO special-casing Unknown to mean "no mapping" is not right here, and can give
+        // confusing/wrong results in cases where there was a mapping found for a typevar, and it
+        // was of type Unknown. We should probably add a bitset or similar to Specialization that
+        // explicitly tells us which typevars are mapped.
         let types: Box<[_]> = self
             .types(db)
             .into_iter()