[SPARK-4982][DOC] `spark.ui.retainedJobs` description is wrong in Spark UI configuration guide

Author: wangxiaojing <u9jing@gmail.com>

Closes #3818 from wangxiaojing/SPARK-4982 and squashes the following commits:

fe2ad5f [wangxiaojing] change stages to jobs

docs/configuration.md

@@ -452,7 +452,7 @@ Apart from these, the following properties are also available, and may be useful
   <td><code>spark.ui.retainedJobs</code></td>
   <td>1000</td>
   <td>
-    How many stages the Spark UI and status APIs remember before garbage
+    How many jobs the Spark UI and status APIs remember before garbage
     collecting.
   </td>
 </tr>