[SPARK-20177] Document about compression way has some little detail ch…

…anges.

## What changes were proposed in this pull request?

Document compression way little detail changes.
1.spark.eventLog.compress add 'Compression will use spark.io.compression.codec.'
2.spark.broadcast.compress add 'Compression will use spark.io.compression.codec.'
3,spark.rdd.compress add 'Compression will use spark.io.compression.codec.'
4.spark.io.compression.codec add 'event log describe'.

eg
Through the documents, I don't know  what is compression mode about 'event log'.

## How was this patch tested?

manual tests

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: 郭小龙 10207633 <guo.xiaolong1@zte.com.cn>

Closes #17498 from guoxiaolongzte/SPARK-20177.