Have a warning comment for AttributeSource.captureState

AttributeSource.captureState is a powerful method that can be used to store and (later on) restore the current state, but it comes with a cost of copying all attributes in this source and sometimes can be a big cost if called multiple times.

We could probably add a warning to indicate this cost, as this method is encapsulated quite well and sometimes people who use it won't be aware of the cost.