missing document about spark.deploy.retainedDrivers

The configuration on spark.deploy.retainedDrivers is undocumented but actually used
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/deploy/master/Master.scala#L60

docs/spark-standalone.md

@@ -156,6 +156,20 @@ SPARK_MASTER_OPTS supports the following system properties:
 
 <table class="table">
 <tr><th>Property Name</th><th>Default</th><th>Meaning</th></tr>
+<tr>
+  <td><code>spark.deploy.retainedApplications</code></td>
+  <td>200</td>
+  <td>
+    The number of completedApps to retain. If this cap is exceeded, then the oldest completedApps will be removed. <br/>
+  </td>
+</tr>
+<tr>
+  <td><code>spark.deploy.retainedDrivers</code></td>
+  <td>200</td>
+  <td>
+    The number of completedDrivers to retain. If this cap is exceeded, then the oldest completedDrivers will be removed. <br/>
+  </td>
+</tr>
 <tr>
   <td><code>spark.deploy.spreadOut</code></td>
   <td>true</td>