[Doc] Improve Python DataFrame documentation

Author: Reynold Xin <rxin@databricks.com>

Closes #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.

1 files changed, 3 insertions, 3 deletions

diff --git a/python/pyspark/sql/functions.py b/python/pyspark/sql/functions.py
index 5873f09ae3..8a478fddf0 100644
--- a/python/pyspark/sql/functions.py
+++ b/python/pyspark/sql/functions.py
@@ -76,7 +76,7 @@ __all__.sort()
 
 
 def countDistinct(col, *cols):
-    """ Return a new Column for distinct count of `col` or `cols`
+    """Returns a new :class:`Column` for distinct count of ``col`` or ``cols``.
 
     >>> df.agg(countDistinct(df.age, df.name).alias('c')).collect()
     [Row(c=2)]
@@ -91,7 +91,7 @@ def countDistinct(col, *cols):
 
 
 def approxCountDistinct(col, rsd=None):
-    """ Return a new Column for approximate distinct count of `col`
+    """Returns a new :class:`Column` for approximate distinct count of ``col``.
 
     >>> df.agg(approxCountDistinct(df.age).alias('c')).collect()
     [Row(c=2)]
@@ -142,7 +142,7 @@ class UserDefinedFunction(object):
 
 
 def udf(f, returnType=StringType()):
-    """Create a user defined function (UDF)
+    """Creates a :class:`Column` expression representing a user defined function (UDF).
 
     >>> from pyspark.sql.types import IntegerType
     >>> slen = udf(lambda s: len(s), IntegerType())