Remove mentioning of TC 6 from docs.
Remove doClusterLog and clusterLogName description
from cluster docs, those no longer exist.

Backport of r1565788 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1565794 13f79535-47bb-0310-9956-ffa450edef68