[SPARK-9641] [DOCS] spark.shuffle.service.port is not documented

Document spark.shuffle.service.{enabled,port}

CC sryza tgravescs
This is pretty minimal; is there more to say here about the service?

Author: Sean Owen <sowen@cloudera.com>

Closes #7991 from srowen/SPARK-9641 and squashes the following commits:

3bb946e [Sean Owen] Add link to docs for setup and config of external shuffle service
2302e01 [Sean Owen] Document spark.shuffle.service.{enabled,port}

docs/configuration.md

@@ -473,6 +473,25 @@ Apart from these, the following properties are also available, and may be useful
     <code>spark.storage.memoryFraction</code>.
   </td>
 </tr>
+<tr>
+  <td><code>spark.shuffle.service.enabled</code></td>
+  <td>false</td>
+  <td>
+    Enables the external shuffle service. This service preserves the shuffle files written by 
+    executors so the executors can be safely removed. This must be enabled if 
+    <code>spark.dynamicAllocation.enabled</code> is "true". The external shuffle service
+    must be set up in order to enable it. See
+    <a href="job-scheduling.html#configuration-and-setup">dynamic allocation 
+    configuration and setup documentation</a> for more information.
+  </td>
+</tr>
+<tr>
+  <td><code>spark.shuffle.service.port</code></td>
+  <td>7337</td>
+  <td>
+    Port on which the external shuffle service will run.
+  </td>
+</tr>
 <tr>
   <td><code>spark.shuffle.sort.bypassMergeThreshold</code></td>
   <td>200</td>