What is "keychainService" in AppConfiguration constructor?

[rnstewart]

The instructions show an example of creating an instance of AppConfiguration using the parameters clientIdentifier, clientSecret, and scopes. But the actual AppConfiguration class requires a fourth parameter called keychainService. What is this? I see no mention of it anywhere in the documentation, and searching elsewhere hasn't helped me in any way.

Can you please clarify this? It would also be nice to update the documentation so others don't run into this problem.