Fix misleading TODO comment in testIterableUnpackingWithGetAttr (#14894)

Follow-up to #14893

See #14892 for more discussion

Author: AlexWaygood

test-data/unit/check-classes.test

@@ -7810,6 +7810,12 @@ class D:
         return C()
 
     def g(self) -> None:
-        # TODO: This is a false positive
+        # iter(x) looks up `__iter__` on the type of x rather than x itself,
+        # so this is correct behaviour.
+        # Instances of C should not be treated as being iterable,
+        # despite having a __getattr__ method
+        # that could allow for arbitrary attributes to be accessed on instances,
+        # since `type(C()).__iter__` still raises AttributeError at runtime,
+        # and that's what matters.
         a, b = self.f()  # E: "C" has no attribute "__iter__" (not iterable)
 [builtins fixtures/tuple.pyi]