[SPARK-7747] [SQL] [DOCS] spark.sql.planner.externalSort

Add documentation for spark.sql.planner.externalSort

Author: Luca Martinetti <luca@luca.io>

Closes #6272 from lucamartinetti/docs-externalsort and squashes the following commits:

985661b [Luca Martinetti] [SPARK-7747] [SQL] [DOCS] Add documentation for spark.sql.planner.externalSort

docs/sql-programming-guide.md

@@ -1785,6 +1785,13 @@ that these options will be deprecated in future release as more optimizations ar
       Configures the number of partitions to use when shuffling data for joins or aggregations.
     </td>
   </tr>
+   <tr>
+    <td><code>spark.sql.planner.externalSort</code></td>
+    <td>false</td>
+    <td>
+      When true, performs sorts spilling to disk as needed otherwise sort each partition in memory.
+    </td>
+  </tr>
 </table>
 
 # Distributed SQL Engine