Extended trait name for anytrait on a HasTraits attribute

First, a code snippet:
```
class B(HasTraits):
s = Int
t = Int

class A(HasTraits):
b = Instance(B)

@on_trait_change('b+')
def fun(self):
print("this should fire when a trait on `b` changes")
```

If my class `A` has a Traits attribute `b` which is itself a HasTraits object, how can I set up a trait change notification for when any attribute on the attribute object `b` changes? This might be [documented here](https://docs.enthought.com/traits/traits_user_manual/notification.html#semantics) but I'm not sure. This issue is to suggest that the documentation be made more clear and to maybe add this usage as an example.