[SPARK-15813] Improve Canceling log message to make it less ambiguous

## What changes were proposed in this pull request?
Add new desired executor number to make the log message less ambiguous.

## How was this patch tested?
This is a trivial change

Author: Peter Ableda <abledapeter@gmail.com>

Closes #13552 from peterableda/patch-1.

yarn/src/main/scala/org/apache/spark/deploy/yarn/YarnAllocator.scala

@@ -354,7 +354,8 @@ private[yarn] class YarnAllocator(
 
     } else if (missing < 0) {
       val numToCancel = math.min(numPendingAllocate, -missing)
-      logInfo(s"Canceling requests for $numToCancel executor containers")
+      logInfo(s"Canceling requests for $numToCancel executor container(s) to have a new desired " +
+        s"total $targetNumExecutors executors.")
 
       val matchingRequests = amClient.getMatchingRequests(RM_REQUEST_PRIORITY, ANY_HOST, resource)
       if (!matchingRequests.isEmpty) {