[SPARK-11860][PYSAPRK][DOCUMENTATION] Invalid argument specification …

…for registerFunction [Python]

Straightforward change on the python doc

Author: Jeff Zhang <zjffdu@apache.org>

Closes #9901 from zjffdu/SPARK-11860.

python/pyspark/sql/context.py

@@ -195,14 +195,15 @@ def range(self, start, end=None, step=1, numPartitions=None):
     @ignore_unicode_prefix
     @since(1.2)
     def registerFunction(self, name, f, returnType=StringType()):
-        """Registers a lambda function as a UDF so it can be used in SQL statements.
+        """Registers a python function (including lambda function) as a UDF
+        so it can be used in SQL statements.
 
         In addition to a name and the function itself, the return type can be optionally specified.
         When the return type is not given it default to a string and conversion will automatically
         be done.  For any other return type, the produced object must match the specified type.
 
         :param name: name of the UDF
-        :param samplingRatio: lambda function
+        :param f: python function
         :param returnType: a :class:`DataType` object
 
         >>> sqlContext.registerFunction("stringLengthString", lambda x: len(x))