[SPARK-19499][SS] Add more notes in the comments of Sink.addBatch()

## What changes were proposed in this pull request?

addBatch method in Sink trait is supposed to be a synchronous method to coordinate with the fault-tolerance design in StreamingExecution (being different with the compute() method in DStream)

We need to add more notes in the comments of this method to remind the developers

## How was this patch tested?

existing tests

Author: CodingCat <zhunansjtu@gmail.com>

Closes #16840 from CodingCat/SPARK-19499.

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/Sink.scala

@@ -31,8 +31,11 @@ trait Sink {
    * this method is called more than once with the same batchId (which will happen in the case of
    * failures), then `data` should only be added once.
    *
-   * Note: You cannot apply any operators on `data` except consuming it (e.g., `collect/foreach`).
+   * Note 1: You cannot apply any operators on `data` except consuming it (e.g., `collect/foreach`).
    * Otherwise, you may get a wrong result.
+   *
+   * Note 2: The method is supposed to be executed synchronously, i.e. the method should only return
+   * after data is consumed by sink successfully.
    */
   def addBatch(batchId: Long, data: DataFrame): Unit
 }