Mention that useful_id uses Zero as a 'blank' value

[PennyJim]

I would like to add some minor clarification that the useful_id that register_on_object_destroyed and on_object_destroyed returns will use `0` instead of nil when there isn't an associated useful id to use.

It may already show that it's not nil-able, but I'm going to be honest in that I kind of overlooked that. Even if you do notice that, it helps to describe the behavior it has when it may otherwise be nil

[asher_sky]

+1
I read the phrase "Useful identifier of the object if it has one" and assumed it would be nil if the object didn't have one, completely overlooking the absence of a question mark next to the return. It would be nice to note that the value is specifically 0 if the object doesn't have a useful identifier