Inconsistency between postconf(5) and IPV6_README

[Pau Amma]

http://www.postfix.org/postconf.5.html#inet_protocols says: 
inet_protocols = all (DEFAULT)
http://www.postfix.org/IPV6_README.html says: inet_protocols = ipv4      
 (DEFAULT: enable IPv4 only)

The inconsistency should be fixed.