Add versionadded for has_ipv6 attribute

Doc/lib/libsocket.tex

@@ -153,6 +153,7 @@ for a few symbols, default values are provided.
 \begin{datadesc}{has_ipv6}
 This constant contains a boolean value which indicates if IPv6 is
 supported on this platform.
+\versionadded{2.3}
 \end{datadesc}
 
 \begin{funcdesc}{getaddrinfo}{host, port\optional{, family, socktype, proto, flags}}