[SPARK-11039][Documentation][Web UI] Document additional ui configurations

Add documentation for configuration:
- spark.sql.ui.retainedExecutions
- spark.streaming.ui.retainedBatches

Author: Nick Pritchard <nicholas.pritchard@falkonry.com>

Closes #9052 from pnpritchard/SPARK-11039.

docs/configuration.md

@@ -554,6 +554,20 @@ Apart from these, the following properties are also available, and may be useful
     How many finished drivers the Spark UI and status APIs remember before garbage collecting.
   </td>
 </tr>
+<tr>
+  <td><code>spark.sql.ui.retainedExecutions</code></td>
+  <td>1000</td>
+  <td>
+    How many finished executions the Spark UI and status APIs remember before garbage collecting.
+  </td>
+</tr>
+<tr>
+  <td><code>spark.streaming.ui.retainedBatches</code></td>
+  <td>1000</td>
+  <td>
+    How many finished batches the Spark UI and status APIs remember before garbage collecting.
+  </td>
+</tr>
 </table>
 
 #### Compression and Serialization