[SPARK-22369][PYTHON][DOCS] Exposes catalog API documentation in PySpark

## What changes were proposed in this pull request?

This PR proposes to add a link from `spark.catalog(..)` to `Catalog` and expose Catalog APIs in PySpark as below:

<img width="740" alt="2017-10-29 12 25 46" src="https://user-images.githubusercontent.com/6477701/32135863-f8e9b040-bc40-11e7-92ad-09c8043a1295.png">

<img width="1131" alt="2017-10-29 12 26 33" src="https://user-images.githubusercontent.com/6477701/32135849-bb257b86-bc40-11e7-9eda-4d58fc1301c2.png">

Note that this is not shown in the list on the top - https://spark.apache.org/docs/latest/api/python/pyspark.sql.html#module-pyspark.sql

<img width="674" alt="2017-10-29 12 30 58" src="https://user-images.githubusercontent.com/6477701/32135854-d50fab16-bc40-11e7-9181-812c56fd22f5.png">

This is basically similar with `DataFrameReader` and `DataFrameWriter`.

## How was this patch tested?

Manually built the doc.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #19596 from HyukjinKwon/SPARK-22369.

python/pyspark/sql/__init__.py

@@ -46,6 +46,7 @@ from pyspark.sql.types import Row
 from pyspark.sql.context import SQLContext, HiveContext, UDFRegistration
 from pyspark.sql.session import SparkSession
 from pyspark.sql.column import Column
+from pyspark.sql.catalog import Catalog
 from pyspark.sql.dataframe import DataFrame, DataFrameNaFunctions, DataFrameStatFunctions
 from pyspark.sql.group import GroupedData
 from pyspark.sql.readwriter import DataFrameReader, DataFrameWriter
@@ -54,7 +55,7 @@ from pyspark.sql.window import Window, WindowSpec
 
 __all__ = [
     'SparkSession', 'SQLContext', 'HiveContext', 'UDFRegistration',
-    'DataFrame', 'GroupedData', 'Column', 'Row',
+    'DataFrame', 'GroupedData', 'Column', 'Catalog', 'Row',
     'DataFrameNaFunctions', 'DataFrameStatFunctions', 'Window', 'WindowSpec',
     'DataFrameReader', 'DataFrameWriter'
 ]

python/pyspark/sql/session.py

@@ -271,6 +271,8 @@ class SparkSession(object):
     def catalog(self):
         """Interface through which the user may create, drop, alter or query underlying
         databases, tables, functions etc.
+
+        :return: :class:`Catalog`
         """
         if not hasattr(self, "_catalog"):
             self._catalog = Catalog(self)