Update configuration.md.

Author: Marcelo Vanzin

docs/configuration.md

@@ -1303,6 +1303,22 @@ Apart from these, the following properties are also available, and may be useful
     not running on YARN and authentication is enabled.
   </td>
 </tr>
+<tr>
+  <td><code>spark.authenticate.enableSaslEncryption</code></td>
+  <td>false</td>
+  <td>
+    Enable encrypted communication when authentication is enabled. This option is currently
+    only supported by the block transfer service.
+  </td>
+</tr>
+<tr>
+  <td><code>spark.network.sasl.serverAlwaysEncrypt</code></td>
+  <td>false</td>
+  <td>
+    Disable unencrypted connections for services that support SASL authentication. This is
+    currently supported by the external shuffle service.
+  </td>
+</tr>
 <tr>
   <td><code>spark.core.connection.ack.wait.timeout</code></td>
   <td>60s</td>