[SPARK-23122][PYSPARK][FOLLOW-UP] Update the docs for UDF Registration

## What changes were proposed in this pull request?

This PR is to update the docs for UDF registration

## How was this patch tested?

N/A

Author: gatorsmile <[email protected]>

Closes #20348 from gatorsmile/testUpdateDoc.

(cherry picked from commit 7328116)
Signed-off-by: gatorsmile <[email protected]>

python/pyspark/sql/udf.py

@@ -199,8 +199,8 @@ def __init__(self, sparkSession):
     @ignore_unicode_prefix
     @since("1.3.1")
     def register(self, name, f, returnType=None):
-        """Registers a Python function (including lambda function) or a user-defined function
-        in SQL statements.
+        """Register a Python function (including lambda function) or a user-defined function
+        as a SQL function.
 
         :param name: name of the user-defined function in SQL statements.
         :param f: a Python function, or a user-defined function. The user-defined function can
@@ -210,6 +210,10 @@ def register(self, name, f, returnType=None):
             be either a :class:`pyspark.sql.types.DataType` object or a DDL-formatted type string.
         :return: a user-defined function.
 
+        To register a nondeterministic Python function, users need to first build
+        a nondeterministic user-defined function for the Python function and then register it
+        as a SQL function.
+
         `returnType` can be optionally specified when `f` is a Python function but not
         when `f` is a user-defined function. Please see below.
 
@@ -297,7 +301,7 @@ def register(self, name, f, returnType=None):
     @ignore_unicode_prefix
     @since(2.3)
     def registerJavaFunction(self, name, javaClassName, returnType=None):
-        """Register a Java user-defined function so it can be used in SQL statements.
+        """Register a Java user-defined function as a SQL function.
 
         In addition to a name and the function itself, the return type can be optionally specified.
         When the return type is not specified we would infer it via reflection.
@@ -334,7 +338,7 @@ def registerJavaFunction(self, name, javaClassName, returnType=None):
     @ignore_unicode_prefix
     @since(2.3)
     def registerJavaUDAF(self, name, javaClassName):
-        """Register a Java user-defined aggregate function so it can be used in SQL statements.
+        """Register a Java user-defined aggregate function as a SQL function.
 
         :param name: name of the user-defined aggregate function
         :param javaClassName: fully qualified name of java class