Updated the comment for SPARK-2162.

A follow up on #1103

@andrewor14

Author: Reynold Xin <[email protected]>

Closes #1117 from rxin/SPARK-2162 and squashes the following commits:

a4231de [Reynold Xin] Updated the comment for SPARK-2162.

Author: rxin

core/src/main/scala/org/apache/spark/storage/BlockManager.scala

@@ -363,11 +363,13 @@ private[spark] class BlockManager(
     val info = blockInfo.get(blockId).orNull
     if (info != null) {
       info.synchronized {
-        // Double check to make sure the block is still there, since removeBlock
-        // method also synchronizes on BlockInfo object, so the block might have
-        // been removed when we actually come here.
+        // Double check to make sure the block is still there. There is a small chance that the
+        // block has been removed by removeBlock (which also synchronizes on the blockInfo object).
+        // Note that this only checks metadata tracking. If user intentionally deleted the block
+        // on disk or from off heap storage without using removeBlock, this conditional check will
+        // still pass but eventually we will get an exception because we can't find the block.
         if (blockInfo.get(blockId).isEmpty) {
-          logDebug(s"Block $blockId had been removed")
+          logWarning(s"Block $blockId had been removed")
           return None
         }