[SPARK-18742][CORE] Clarify that user-defined BroadcastFactory is not supported

## What changes were proposed in this pull request?
After SPARK-12588 Remove HTTPBroadcast [1], the one and only implementation of BroadcastFactory is TorrentBroadcastFactory and the spark.broadcast.factory conf has removed.

however the scaladoc says [2]:

/**
 * An interface for all the broadcast implementations in Spark (to allow
 * multiple broadcast implementations). SparkContext uses a user-specified
 * BroadcastFactory implementation to instantiate a particular broadcast for the
 * entire Spark job.
 */

so we should modify the comment that SparkContext will not use a  user-specified BroadcastFactory implementation

[1] https://issues.apache.org/jira/browse/SPARK-12588
[2] https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/broadcast/BroadcastFactory.scala#L25-L30

## How was this patch tested?
unit test added

Author: root <root@iZbp1gsnrlfzjxh82cz80vZ.(none)>
Author: windpiger <songjun@outlook.com>

Closes #16173 from windpiger/addBroadFactoryConf.

core/src/main/scala/org/apache/spark/broadcast/BroadcastFactory.scala

@@ -24,9 +24,8 @@ import org.apache.spark.SparkConf
 
 /**
  * An interface for all the broadcast implementations in Spark (to allow
- * multiple broadcast implementations). SparkContext uses a user-specified
- * BroadcastFactory implementation to instantiate a particular broadcast for the
- * entire Spark job.
+ * multiple broadcast implementations). SparkContext uses a BroadcastFactory
+ * implementation to instantiate a particular broadcast for the entire Spark job.
  */
 private[spark] trait BroadcastFactory {