[SPARK-2524] missing document about spark.deploy.retainedDrivers

https://issues.apache.org/jira/browse/SPARK-2524
The configuration on spark.deploy.retainedDrivers is undocumented but actually used
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/deploy/master/Master.scala#L60
This language is specific to internal naming. It might be better to say something like this:

```
The maximum number of completed applications to display. Older applications will be dropped from the UI to maintain this limit.
```

And likewise below.