small doco update specifying that Checker-Version can't be changed or removed
small doco update specifying that Checker-Version can't be changed or removed

git-svn-id: e9fe9b84e32fc5e57b7d09c40cd78b14ea28ed71@56952 13f79535-47bb-0310-9956-ffa450edef68

apache/spamassassin,apache/spamassassin,apache/spamassassin,apache/spamassassin,apache/spamassassin