add java doc for task state

src/main/java/com/amazon/opendistroforelasticsearch/ad/model/ADTaskState.java

@@ -15,6 +15,40 @@
 
 package com.amazon.opendistroforelasticsearch.ad.model;
 
+/**
+ * AD task states.
+ * <ul>
+ * <li><code>CREATED</code>:
+ *     When user start a historical detector, we will create one task to track the detector
+ *     execution and set its state as CREATED
+ *
+ * <li><code>INIT</code>:
+ *     After task created, coordinate node will gather all eligible node’s state and dispatch
+ *     task to the worker node with lowest load. When the worker node receives the request,
+ *     it will set the task state as INIT immediately, then start to run cold start to train
+ *     RCF model. We will track the initialization progress in task.
+ *     Init_Progress=ModelUpdates/MinSampleSize
+ *
+ * <li><code>RUNNING</code>:
+ *     If RCF model gets enough data points and passed training, it will start to detect data
+ *     normally and output positive anomaly scores. Once the RCF model starts to output positive
+ *     anomaly score, we will set the task state as RUNNING and init progress as 100%. We will
+ *     track task running progress in task: Task_Progress=DetectedPieces/AllPieces
+ *
+ * <li><code>FINISHED</code>:
+ *     When all historical data detected, we set the task state as FINISHED and task progress
+ *     as 100%.
+ *
+ * <li><code>STOPPED</code>:
+ *     User can cancel a running task by stopping detector, for example, user want to tune
+ *     feature and reran and don’t want current task run any more. When a historical detector
+ *     stopped, we will mark the task flag cancelled as true, when run next piece, we will
+ *     check this flag and stop the task. Then task stopped, will set its state as STOPPED
+ *
+ * <li><code>FAILED</code>:
+ *     If any exception happen, we will set task state as FAILED
+ * </ul>
+ */
 public enum ADTaskState {
     CREATED,
     INIT,