[SPARK-3035] Wrong example with SparkContext.addFile

https://issues.apache.org/jira/browse/SPARK-3035

fix for wrong document.

Author: iAmGhost <kdh7807@gmail.com>

Closes #1942 from iAmGhost/master and squashes the following commits:

487528a [iAmGhost] [SPARK-3035] Wrong example with SparkContext.addFile fix for wrong document.
[SPARK-1065] [PySpark] improve supporting for large broadcast

Passing large object by py4j is very slow (cost much memory), so pass broadcast objects via files (similar to parallelize()).

Add an option to keep object in driver (it's False by default) to save memory in driver.

Author: Davies Liu <davies.liu@gmail.com>

Closes #1912 from davies/broadcast and squashes the following commits:

e06df4a [Davies Liu] load broadcast from disk in driver automatically
db3f232 [Davies Liu] fix serialization of accumulator
631a827 [Davies Liu] Merge branch 'master' into broadcast
c7baa8c [Davies Liu] compress serrialized broadcast and command
9a7161f [Davies Liu] fix doc tests
e93cf4b [Davies Liu] address comments: add test
6226189 [Davies Liu] improve large broadcast
In the stop method of ConnectionManager to cancel the ackTimeoutMonitor

cc JoshRosen sarutak

Author: GuoQiang Li <witgo@qq.com>

Closes #1989 from witgo/cancel_ackTimeoutMonitor and squashes the following commits:

4a700fa [GuoQiang Li] In the stop method of ConnectionManager to cancel the ackTimeoutMonitor
[SPARK-3077][MLLIB] fix some chisq-test

- promote nullHypothesis field in ChiSqTestResult to TestResult. Every test should have a null hypothesis
- correct null hypothesis statement for independence test
- p-value: 0.01 -> 0.1

Author: Xiangrui Meng <meng@databricks.com>

Closes #1982 from mengxr/fix-chisq and squashes the following commits:

5f0de02 [Xiangrui Meng] make ChiSqTestResult constructor package private
bc74ea1 [Xiangrui Meng] update chisq-test