[SPARK-4330][Doc] Link to proper URL for YARN overview

In running-on-yarn.md, a link to YARN overview is here.
But the URL is to YARN alpha's.
It should be stable's.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #3196 from sarutak/SPARK-4330 and squashes the following commits:

30baa21 [Kousuke Saruta] Fixed running-on-yarn.md to point proper URL for YARN

(cherry picked from commit 3c07b8f08240bafcdff5d174989fb433f4bc80b6)
Signed-off-by: Matei Zaharia <matei@databricks.com>
[BRANCH-1.1][SPARK-2652] change the default spark.serializer in pyspark back to Kryo

This reverts #2916 . We shouldn't change the default settings in a minor release. JoshRosen davies

Author: Xiangrui Meng <meng@databricks.com>

Closes #3187 from mengxr/SPARK-2652-1.1 and squashes the following commits:

372166b [Xiangrui Meng] change the default spark.serializer in pyspark back to Kryo
[branch-1.1][SPARK-3990] add a note on ALS usage

Because we switched back to Kryo in #3187 , we need to leave a note about the workaround.

Author: Xiangrui Meng <meng@databricks.com>

Closes #3190 from mengxr/SPARK-3990-1.1 and squashes the following commits:

d4818f3 [Xiangrui Meng] fix python style
53725b0 [Xiangrui Meng] add a note about SPARK-3990
56ad70e [Xiangrui Meng] add a note about SPARK-3990