[SPARK-16772][PYTHON][DOCS] Fix API doc references to UDFRegistration + Update "important classes"

## 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.

python/docs/index.rst

@@ -50,4 +50,3 @@ Indices and tables
 ==================
 
 * :ref:`search`
-

python/docs/pyspark.sql.rst

@@ -8,14 +8,12 @@ Module Context
     :members:
     :undoc-members:
 
-
 pyspark.sql.types module
 ------------------------
 .. automodule:: pyspark.sql.types
     :members:
     :undoc-members:
 
-
 pyspark.sql.functions module
 ----------------------------
 .. automodule:: pyspark.sql.functions

python/pyspark/sql/__init__.py

@@ -18,7 +18,7 @@
 """
 Important classes of Spark SQL and DataFrames:
 
-    - :class:`pyspark.sql.SQLContext`
+    - :class:`pyspark.sql.SparkSession`
       Main entry point for :class:`DataFrame` and SQL functionality.
     - :class:`pyspark.sql.DataFrame`
       A distributed collection of data grouped into named columns.
@@ -26,8 +26,6 @@ Important classes of Spark SQL and DataFrames:
       A column expression in a :class:`DataFrame`.
     - :class:`pyspark.sql.Row`
       A row of data in a :class:`DataFrame`.
-    - :class:`pyspark.sql.HiveContext`
-      Main entry point for accessing data stored in Apache Hive.
     - :class:`pyspark.sql.GroupedData`
       Aggregation methods, returned by :func:`DataFrame.groupBy`.
     - :class:`pyspark.sql.DataFrameNaFunctions`
@@ -45,7 +43,7 @@ from __future__ import absolute_import
 
 
 from pyspark.sql.types import Row
-from pyspark.sql.context import SQLContext, HiveContext
+from pyspark.sql.context import SQLContext, HiveContext, UDFRegistration
 from pyspark.sql.session import SparkSession
 from pyspark.sql.column import Column
 from pyspark.sql.dataframe import DataFrame, DataFrameNaFunctions, DataFrameStatFunctions
@@ -55,7 +53,8 @@ from pyspark.sql.window import Window, WindowSpec
 
 
 __all__ = [
-    'SparkSession', 'SQLContext', 'HiveContext', 'DataFrame', 'GroupedData', 'Column',
-    'Row', 'DataFrameNaFunctions', 'DataFrameStatFunctions', 'Window', 'WindowSpec',
+    'SparkSession', 'SQLContext', 'HiveContext', 'UDFRegistration',
+    'DataFrame', 'GroupedData', 'Column', 'Row',
+    'DataFrameNaFunctions', 'DataFrameStatFunctions', 'Window', 'WindowSpec',
     'DataFrameReader', 'DataFrameWriter'
 ]