SPARK-1299: making comments of RDD.doCheckpoint consistent with its usage

https://spark-project.atlassian.net/browse/SPARK-1299

another thing I found occasionally, the comments of function is saying that

```
/**
Performs the checkpointing of this RDD by saving this. It is called by the DAGScheduler
after a job using this RDD has completed (therefore the RDD has been materialized and
potentially stored in memory). doCheckpoint() is called recursively on the parent RDDs.
*/

```

actually this function is called in SparkContext.runJob

we can either change the comments or call it in DAGScheduler, I personally prefer the later one, as this calling seems like an auto-checkpoint , better put it in a non-user-facing component