[SPARK-8641][SPARK-12455][SQL] Native Spark Window functions - Follow-up (docs & tests)

This PR is a follow-up for PR https://github.com/apache/spark/pull/9819. It adds documentation for the window functions and a couple of NULL tests.

The documentation was largely based on the documentation in (the source of)  Hive and Presto:
* https://prestodb.io/docs/current/functions/window.html
* https://cwiki.apache.org/confluence/display/Hive/LanguageManual+WindowingAndAnalytics

I am not sure if we need to add the licenses of these two projects to the licenses directory. They are both under the ASL. srowen any thoughts?

cc yhuai

Author: Herman van Hovell <hvanhovell@questtec.nl>

Closes #10402 from hvanhovell/SPARK-8641-docs.