Add docs for rpc configurations

Author: zsxwing

docs/configuration.md

@@ -982,6 +982,34 @@ Apart from these, the following properties are also available, and may be useful
     This is only relevant for the Spark shell.
   </td>
 </tr>
+<tr>
+  <td><code>spark.rpc.numRetries</code></td>
+  <td>3</td>
+    How many times for an RPC ask operation to retry before giving up.
+  <td>
+  </td>
+</tr>
+<tr>
+  <td><code>spark.rpc.retry.wait</code></td>
+  <td>3s</td>
+  <td>
+    How long for an RPC ask operation to wait before starting the next retry.
+  </td>
+</tr>
+<tr>
+  <td><code>spark.rpc.askTimeout</code></td>
+  <td>30s</td>
+  <td>
+    How long for an RPC ask operation to wait before timing out.
+  </td>
+</tr>
+<tr>
+  <td><code>spark.rpc.lookupTimeout</code></td>
+  <td>30s</td>
+    How long for an RPC remote endpoint lookup operation to wait before timing out.
+  <td>
+  </td>
+</tr>
 </table>
 
 #### Scheduling