[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.