Adjusted docs regarding EDNS0 dns option to refer to RFC 6891 instead of RFC 2671bis draft, refine text, reduce conservative default from 1240 to 1220 bytes

git-svn-id: https://svn.apache.org/repos/asf/spamassassin/trunk@1540791 13f79535-47bb-0310-9956-ffa450edef68