[SPARK-4982][DOC] `spark.ui.retainedJobs` description is wrong in Spark UI configuration guide

Author: wangxiaojing <u9jing@gmail.com>

Closes #3818 from wangxiaojing/SPARK-4982 and squashes the following commits:

fe2ad5f [wangxiaojing] change stages to jobs

(cherry picked from commit 6645e52580747990321e22340ae742f26d2f2504)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>
SPARK-4968: takeOrdered to skip reduce step in case mappers return no partitions

takeOrdered should skip reduce step in case mapped RDDs have no partitions. This prevents the mentioned exception :

4. run query
SELECT * FROM testTable WHERE market = 'market2' ORDER BY End_Time DESC LIMIT 100;
Error trace
java.lang.UnsupportedOperationException: empty collection
at org.apache.spark.rdd.RDD$$anonfun$reduce$1.apply(RDD.scala:863)
at org.apache.spark.rdd.RDD$$anonfun$reduce$1.apply(RDD.scala:863)
at scala.Option.getOrElse(Option.scala:120)
at org.apache.spark.rdd.RDD.reduce(RDD.scala:863)
at org.apache.spark.rdd.RDD.takeOrdered(RDD.scala:1136)

Author: Yash Datta <Yash.Datta@guavus.com>

Closes #3830 from saucam/fix_takeorder and squashes the following commits:

5974d10 [Yash Datta] SPARK-4968: takeOrdered to skip reduce step in case mappers return no partitions

(cherry picked from commit 9bc0df6804f241aff24520d9c6ec54d9b11f5785)
Signed-off-by: Reynold Xin <rxin@databricks.com>