Document how listen_addresses can do only IPv4 or IPv6.

doc/src/sgml/config.sgml

@@ -324,7 +324,9 @@ SET ENABLE_SEQSCAN TO OFF;
          to listen for connections from client applications.
          The value takes the form of a comma-separated list of host names
          and/or numeric IP addresses.  The special entry <literal>*</>
-         corresponds to all available IP interfaces.
+         corresponds to all available IP interfaces.  The entry
+         <literal>0.0.0.0</> allows listening for all IPv4 addresses and
+         <literal>::</> allows listening for all IPv6 addresses.
          If the list is empty, the server does not listen on any IP interface
          at all, in which case only Unix-domain sockets can be used to connect
          to it.