[SPARK-17449][DOCUMENTATION] Relation between heartbeatInterval and…

## What changes were proposed in this pull request?

The relation between spark.network.timeout and spark.executor.heartbeatInterval should be mentioned in the document.

… network timeout]

Author: Jagadeesan <as2@us.ibm.com>

Closes #15042 from jagadeesanas2/SPARK-17449.

core/src/main/scala/org/apache/spark/HeartbeatReceiver.scala

@@ -32,6 +32,7 @@ import org.apache.spark.util._
  * A heartbeat from executors to the driver. This is a shared message used by several internal
  * components to convey liveness or execution information for in-progress tasks. It will also
  * expire the hosts that have not heartbeated for more than spark.network.timeout.
+ * spark.executor.heartbeatInterval should be significantly less than spark.network.timeout.
  */
 private[spark] case class Heartbeat(
     executorId: String,

docs/configuration.md

@@ -987,7 +987,8 @@ Apart from these, the following properties are also available, and may be useful
     <td>10s</td>
     <td>Interval between each executor's heartbeats to the driver.  Heartbeats let
     the driver know that the executor is still alive and update it with metrics for in-progress
-    tasks.</td>
+    tasks. spark.executor.heartbeatInterval should be significantly less than
+    spark.network.timeout</td>
 </tr>
 <tr>
   <td><code>spark.files.fetchTimeout</code></td>