improve the doc for "spark.memory.offHeap.size"

Author: CodingCat

docs/configuration.md

@@ -769,7 +769,7 @@ Apart from these, the following properties are also available, and may be useful
   <td><code>spark.memory.offHeap.size</code></td>
   <td>0</td>
   <td>
-    The absolute amount of memory in bytes which can be used for off-heap allocation.
+    The absolute amount of memory (in terms by bytes) which can be used for off-heap allocation.
     This setting has no impact on heap memory usage, so if your executors' total memory consumption must fit within some hard limit then be sure to shrink your JVM heap size accordingly.
     This must be set to a positive value when <code>spark.memory.offHeap.enabled=true</code>.
   </td>