[Docs] Add missing <code> tags (minor)

These configs looked inconsistent from the rest.

Author: Andrew Or <andrewor14@gmail.com>

Closes #1936 from andrewor14/docs-code and squashes the following commits:

15f578a [Andrew Or] Add <code> tag
[SPARK-2995][MLLIB] add ALS.setIntermediateRDDStorageLevel

As mentioned in SPARK-2465, using `MEMORY_AND_DISK_SER` for user/product in/out links together with `spark.rdd.compress=true` can help reduce the space requirement by a lot, at the cost of speed. It might be useful to add this option so people can run ALS on much bigger datasets.

Another option for the method name is `setIntermediateRDDStorageLevel`.

Author: Xiangrui Meng <meng@databricks.com>

Closes #1913 from mengxr/als-storagelevel and squashes the following commits:

d942017 [Xiangrui Meng] rename to setIntermediateRDDStorageLevel
7550029 [Xiangrui Meng] add ALS.setIntermediateDataStorageLevel