Update comments to better explain the IDs

Author: jinliu9508

OneSignalSDK/onesignal/core/src/main/java/com/onesignal/user/IUserManager.kt

@@ -27,12 +27,15 @@ interface IUserManager {
     val pushSubscription: IPushSubscription
 
     /**
-     * onesignalId, empty if this is current unavailable
+     * The UUID generated by OneSignal to represent a user, empty if this is currently unavailable
      */
     val onesignalId: String
 
     /**
-     * externalId, empty if this is current unavailable
+     * The External ID is OneSignal's default and recommended alias label. This should be the main
+     * identifier you use to identify users. It is set when calling the [OneSignal.login] method.
+     *
+     * This is empty if the External ID has not been set.
      */
     val externalId: String
 
@@ -153,6 +156,9 @@ interface IUserManager {
     /**
      * Add an observer to the user state, allowing the provider to be
      * notified whenever 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.
      */
     fun addObserver(observer: IUserStateObserver)