[SPARK-30387] Improving stop hook log message

### What changes were proposed in this pull request?

ShutdownHook of YarnClientSchedulerBackend prints just "Stopped" which can be improved to "YarnClientSchedulerBackend Stopped" for better understanding.

### Why are the changes needed?

While stopping or gracefully exiting the spark-shell/spark-sql --master yarn, only printing `stopped` is useless.
### Does this PR introduce any user-facing change?

Yes. Log info message change.

### How was this patch tested?

Manually

Closes #27049 from jobitmathew/imp_stop_message.

Authored-by: Jobit Mathew <jobit.mathew@huawei.com>
Signed-off-by: Sean Owen <srowen@gmail.com>

resource-managers/yarn/src/main/scala/org/apache/spark/scheduler/cluster/YarnClientSchedulerBackend.scala

@@ -164,7 +164,7 @@ private[spark] class YarnClientSchedulerBackend(
 
     super.stop()
     client.stop()
-    logInfo("Stopped")
+    logInfo("YARN client scheduler backend Stopped")
   }
 
   override protected def updateDelegationTokens(tokens: Array[Byte]): Unit = {