[SPARK-7351] [STREAMING] [DOCS] Add spark.streaming.ui.retainedBatches to docs

The default value will be changed to `1000` in #5533. So here I just used `1000`.

Author: zsxwing <zsxwing@gmail.com>

Closes #5899 from zsxwing/SPARK-7351 and squashes the following commits:

e1ec515 [zsxwing] [SPARK-7351][Streaming][Docs] Add spark.streaming.ui.retainedBatches to docs

docs/configuration.md

@@ -1474,6 +1474,13 @@ Apart from these, the following properties are also available, and may be useful
     the new Kafka direct stream API.
   </td>
 </tr>
+<tr>
+  <td><code>spark.streaming.ui.retainedBatches</code></td>
+  <td>1000</td>
+  <td>
+    How many batches the Spark Streaming UI and status APIs remember before garbage collecting.
+  </td>
+</tr>
 </table>
 
 #### Cluster Managers