## Proposed Changes

* Update the list of "important classes" in `pyspark.sql` to match 2.0.
* Fix references to `UDFRegistration` so that the class shows up in the docs. It currently [doesn't](http://spark.apache.org/docs/latest/api/python/pyspark.sql.html).
* Remove some unnecessary whitespace in the Python RST doc files.

I reused the [existing JIRA](https://issues.apache.org/jira/browse/SPARK-16772) I created last week for similar API doc fixes.

## How was this patch tested?

* I ran `lint-python` successfully.
* I ran `make clean build` on the Python docs and confirmed the results are as expected locally in my browser.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #14496 from nchammas/SPARK-16772-UDFRegistration.