[red-knot] update TODO comment in mdtest

crates/red_knot_python_semantic/resources/mdtest/binary/integers.md

@@ -10,9 +10,8 @@ reveal_type(-3 // 3)  # revealed: Literal[-1]
 reveal_type(-3 / 3)  # revealed: float
 reveal_type(5 % 3)  # revealed: Literal[2]
 
-# TODO: This should emit an unsupported-operator error but we don't currently
-#  verify that the actual parameter to `int.__add__` matches the declared
-#  formal parameter type.
+# TODO: Should emit `unsupported-operator` but we don't understand the bases of `str`, so we think
+#       it inherits `Unknown`, so we think `str.__radd__` is `Unknown` instead of nonexistent.
 reveal_type(2 + "f")  # revealed: Unknown
 
 def lhs(x: int):