We aren't really doing API documentation at the moment because of the amount of code churn. I'll make sure it gets documented correctly when we do API docs. Note that we have the same semantics in the old stack for methods like CreateIfNotExists, which is the moral equivalent of this method. Also, it's a pattern used elsewhere--for example, indicating whether a call to Remove actually removed an item or was a no-op because the item didn't exist in the list.