[nit] add note about OutcomeEventParams.timestamp

* When an outcome is created, its `timestamp` should start out as zero.
* When the outcome request is sent and fails, the SDK will save and retry the outcome, then it will save the current `timestamp` for future sending

Author: nan-li

OneSignalSDK/onesignal/core/src/main/java/com/onesignal/session/internal/outcomes/impl/OutcomeEventParams.kt

@@ -8,7 +8,7 @@ internal class OutcomeEventParams constructor(
     val outcomeSource: OutcomeSource?, // This field is optional
     var weight: Float, // This field is optional.
     var sessionTime: Long, // This field is optional
-    var timestamp: Long,
+    var timestamp: Long, // This should start out as zero
 ) {
     @Throws(JSONException::class)
     fun toJSONObject(): JSONObject {