[SPARK-18051][SPARK CORE] fix bug of custom PartitionCoalescer causing serialization exception

## What changes were proposed in this pull request?

add a require check in `CoalescedRDD` to make sure the passed in `partitionCoalescer` to be `serializable`.
and update the document for api `RDD.coalesce`

## How was this patch tested?

Manual.(test code in jira [SPARK-18051])

Author: WeichenXu <WeichenXu123@outlook.com>

Closes #15587 from WeichenXu123/fix_coalescer_bug.