fix spark.yarn.max.executor.failures explaination

According to 
```scala
  private val maxNumExecutorFailures = sparkConf.getInt("spark.yarn.max.executor.failures",
    sparkConf.getInt("spark.yarn.max.worker.failures", math.max(args.numExecutors * 2, 3)))
```
default value should be numExecutors * 2, with minimum of 3,  and it's same to the config 
`spark.yarn.max.worker.failures`
we haven't been listing deprecated configs as we don't want people to continue to use them.

It would be nice if we had somewhere people could go to see the list of deprecated configs and new mappings though. But that is a separate jira.