pythongh-64414: mention AF_INET6 and IPv6 in socketserver docs. (pythonGH-129866)

mention AF_INET6 and IPv6 in socketserver docs.
(cherry picked from commit 5ce70ad)

Co-authored-by: Gregory P. Smith <[email protected]>

Author: gpshead

Doc/library/socketserver.rst

@@ -260,8 +260,11 @@ Server Objects
 
    .. attribute:: address_family
 
-      The family of protocols to which the server's socket belongs.
-      Common examples are :const:`socket.AF_INET` and :const:`socket.AF_UNIX`.
+      The family of protocols to which the server's socket belongs.  Common
+      examples are :const:`socket.AF_INET`, :const:`socket.AF_INET6`, and
+      :const:`socket.AF_UNIX`.  Subclass the TCP or UDP server classes in this
+      module with class attribute ``address_family = AF_INET6`` set if you
+      want IPv6 server classes.
 
 
    .. attribute:: RequestHandlerClass