[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>