[SPARK-2945][YARN][Doc]add doc for spark.executor.instances

https://issues.apache.org/jira/browse/SPARK-2945

spark.executor.instances works. As this JIRA recommended, we should add docs for this common config.

Author: WangTaoTheTonic <wangtao111@huawei.com>

Closes #4350 from WangTaoTheTonic/SPARK-2945 and squashes the following commits:

4c3913a [WangTaoTheTonic] not compatible with dynamic allocation
5fa9c46 [WangTaoTheTonic] add doc for spark.executor.instances

docs/running-on-yarn.md

@@ -104,6 +104,13 @@ Most of the configs are the same for Spark on YARN as for other deployment modes
     Comma-separated list of files to be placed in the working directory of each executor.
   </td>
 </tr>
+<tr>
+ <td><code>spark.executor.instances</code></td>
+  <td>2</td>
+  <td>
+    The number of executors. Note that this property is incompatible with <code>spark.dynamicAllocation.enabled</code>.
+  </td>
+</tr>
 <tr>
  <td><code>spark.yarn.executor.memoryOverhead</code></td>
   <td>executorMemory * 0.07, with minimum of 384 </td>