Add in-code documentation on user state observer

* Add a callout on the usage of the user state observer.
* App developers should be checking the `externalId` when they retreive the `onesignalId`. This is to make sure they are grabbing the onesignalId for the user that they expect.

iOS_SDK/OneSignalSDK/OneSignalUser/Source/OneSignalUserManagerImpl.swift

@@ -50,7 +50,10 @@ import OneSignalNotifications
     var pushSubscription: OSPushSubscription { get }
     var onesignalId: String? { get }
     var externalId: String? { get }
-    // User State Observer
+    /**
+     Add an observer to the user state, allowing the provider to be notified when the user state has changed. 
+     Important: When using the observer to retrieve the `onesignalId`, check the `externalId` as well to confirm the values are associated with the expected user.
+     */
     func addObserver(_ observer: OSUserStateObserver)
     func removeObserver(_ observer: OSUserStateObserver)
     // Aliases