[Doc] Improve Python DataFrame documentation

Author: Reynold Xin <[email protected]>

Closes apache#5287 from rxin/pyspark-df-doc-cleanup-context and squashes the following commits:

1841b60 [Reynold Xin] Lint.
f2007f1 [Reynold Xin] functions and types.
bc3b72b [Reynold Xin] More improvements to DataFrame Python doc.
ac1d4c0 [Reynold Xin] Bug fix.
b163365 [Reynold Xin] Python fix. Added Experimental flag to DataFrameNaFunctions.
608422d [Reynold Xin] [Doc] Cleanup context.py Python docs.

Author: rxin

python/pyspark/sql/__init__.py

@@ -16,7 +16,7 @@
 #
 
 """
-public classes of Spark SQL:
+Important classes of Spark SQL and DataFrames:
 
     - L{SQLContext}
       Main entry point for :class:`DataFrame` and SQL functionality.
@@ -34,6 +34,8 @@
       Methods for handling missing data (null values).
     - L{functions}
       List of built-in functions available for :class:`DataFrame`.
+    - L{types}
+      List of data types available.
 """
 
 from pyspark.sql.context import SQLContext, HiveContext