SPARK-1178: missing document of spark.scheduler.revive.interval

https://spark-project.atlassian.net/browse/SPARK-1178

The configuration on spark.scheduler.revive.interval is undocumented but actually used

https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedSchedulerBackend.scala#L64

Author: CodingCat <zhunansjtu@gmail.com>

Closes apache#74 from CodingCat/SPARK-1178 and squashes the following commits:

783ec69 [CodingCat] missing document of spark.scheduler.revive.interval

docs/configuration.md

@@ -201,6 +201,13 @@ Apart from these, the following properties are also available, and may be useful
     multi-user services.
   </td>
 </tr>
+<tr>
+  <td>spark.scheduler.revive.interval</td>
+  <td>1000</td>
+  <td>
+    The interval length for the scheduler to revive the worker resource offers to run tasks. (in milliseconds)
+  </td>
+</tr>
 <tr>
   <td>spark.reducer.maxMbInFlight</td>
   <td>48</td>